diff --git a/crates/wiggle/generate/src/config.rs b/crates/wiggle/generate/src/config.rs
index 0d4ada6402..5b09f9dee6 100644
--- a/crates/wiggle/generate/src/config.rs
+++ b/crates/wiggle/generate/src/config.rs
@@ -1,3 +1,4 @@
+use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
 use proc_macro2::Span;
@@ -12,12 +13,14 @@ use syn::{
 pub struct Config {
     pub witx: WitxConf,
     pub ctx: CtxConf,
+    pub errors: ErrorConf,
 }
 
 #[derive(Debug, Clone)]
 pub enum ConfigField {
     Witx(WitxConf),
     Ctx(CtxConf),
+    Error(ErrorConf),
 }
 
 impl ConfigField {
@@ -25,7 +28,8 @@ impl ConfigField {
         match ident {
             "witx" => Ok(ConfigField::Witx(value.parse()?)),
             "ctx" => Ok(ConfigField::Ctx(value.parse()?)),
-            _ => Err(Error::new(err_loc, "expected `witx` or `ctx`")),
+            "errors" => Ok(ConfigField::Error(value.parse()?)),
+            _ => Err(Error::new(err_loc, "expected `witx`, `ctx`, or `errors`")),
         }
     }
 }
@@ -42,14 +46,27 @@ impl Config {
     pub fn build(fields: impl Iterator<Item = ConfigField>, err_loc: Span) -> Result<Self> {
         let mut witx = None;
         let mut ctx = None;
+        let mut errors = None;
         for f in fields {
             match f {
                 ConfigField::Witx(c) => {
+                    if witx.is_some() {
+                        return Err(Error::new(err_loc, "duplicate `witx` field"));
+                    }
                     witx = Some(c);
                 }
                 ConfigField::Ctx(c) => {
+                    if ctx.is_some() {
+                        return Err(Error::new(err_loc, "duplicate `ctx` field"));
+                    }
                     ctx = Some(c);
                 }
+                ConfigField::Error(c) => {
+                    if errors.is_some() {
+                        return Err(Error::new(err_loc, "duplicate `errors` field"));
+                    }
+                    errors = Some(c);
+                }
             }
         }
         Ok(Config {
@@ -59,6 +76,7 @@ impl Config {
             ctx: ctx
                 .take()
                 .ok_or_else(|| Error::new(err_loc, "`ctx` field required"))?,
+            errors: errors.take().unwrap_or_default(),
         })
     }
 }
@@ -113,3 +131,69 @@ impl Parse for CtxConf {
         })
     }
 }
+
+#[derive(Clone, Default, Debug)]
+/// Map from abi error type to rich error type
+pub struct ErrorConf(HashMap<Ident, ErrorConfField>);
+
+impl ErrorConf {
+    pub fn iter(&self) -> impl Iterator<Item = (&Ident, &ErrorConfField)> {
+        self.0.iter()
+    }
+}
+
+impl Parse for ErrorConf {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let content;
+        let _ = braced!(content in input);
+        let items: Punctuated<ErrorConfField, Token![,]> =
+            content.parse_terminated(Parse::parse)?;
+        let mut m = HashMap::new();
+        for i in items {
+            match m.insert(i.abi_error.clone(), i.clone()) {
+                None => {}
+                Some(prev_def) => {
+                    return Err(Error::new(
+                        i.err_loc,
+                        format!(
+                        "duplicate definition of rich error type for {:?}: previously defined at {:?}",
+                        i.abi_error, prev_def.err_loc,
+                    ),
+                    ))
+                }
+            }
+        }
+        Ok(ErrorConf(m))
+    }
+}
+
+#[derive(Clone)]
+pub struct ErrorConfField {
+    pub abi_error: Ident,
+    pub rich_error: syn::Path,
+    pub err_loc: Span,
+}
+
+impl std::fmt::Debug for ErrorConfField {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("ErrorConfField")
+            .field("abi_error", &self.abi_error)
+            .field("rich_error", &"(...)")
+            .field("err_loc", &self.err_loc)
+            .finish()
+    }
+}
+
+impl Parse for ErrorConfField {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let err_loc = input.span();
+        let abi_error = input.parse::<Ident>()?;
+        let _arrow: Token![=>] = input.parse()?;
+        let rich_error = input.parse::<syn::Path>()?;
+        Ok(ErrorConfField {
+            abi_error,
+            rich_error,
+            err_loc,
+        })
+    }
+}
diff --git a/crates/wiggle/generate/src/error_transform.rs b/crates/wiggle/generate/src/error_transform.rs
new file mode 100644
index 0000000000..80cb56f925
--- /dev/null
+++ b/crates/wiggle/generate/src/error_transform.rs
@@ -0,0 +1,72 @@
+use crate::config::ErrorConf;
+use anyhow::{anyhow, Error};
+use proc_macro2::TokenStream;
+use quote::quote;
+use std::collections::HashMap;
+use std::rc::Rc;
+use witx::{Document, Id, NamedType, TypeRef};
+
+pub struct ErrorTransform {
+    m: Vec<UserErrorType>,
+}
+
+impl ErrorTransform {
+    pub fn new(conf: &ErrorConf, doc: &Document) -> Result<Self, Error> {
+        let mut richtype_identifiers = HashMap::new();
+        let m = conf.iter().map(|(ident, field)|
+            if let Some(abi_type) = doc.typename(&Id::new(ident.to_string())) {
+                    if let Some(ident) = field.rich_error.get_ident() {
+                        if let Some(prior_def) = richtype_identifiers.insert(ident.clone(), field.err_loc.clone())
+                         {
+                            return Err(anyhow!(
+                                    "duplicate rich type identifier of {:?} not allowed. prior definition at {:?}",
+                                    ident, prior_def
+                                ));
+                        }
+                        Ok(UserErrorType {
+                            abi_type,
+                            rich_type: field.rich_error.clone(),
+                            method_fragment: ident.to_string()
+                        })
+                    } else {
+                        return Err(anyhow!(
+                            "rich error type must be identifier for now - TODO add ability to provide a corresponding identifier: {:?}",
+                            field.err_loc
+                        ))
+                    }
+                }
+                else { Err(anyhow!("No witx typename \"{}\" found", ident.to_string())) }
+        ).collect::<Result<Vec<_>, Error>>()?;
+        Ok(Self { m })
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = &UserErrorType> {
+        self.m.iter()
+    }
+
+    pub fn for_abi_error(&self, tref: &TypeRef) -> Option<&UserErrorType> {
+        match tref {
+            TypeRef::Name(nt) => self.m.iter().find(|u| u.abi_type.name == nt.name),
+            TypeRef::Value { .. } => None,
+        }
+    }
+}
+
+pub struct UserErrorType {
+    abi_type: Rc<NamedType>,
+    rich_type: syn::Path,
+    method_fragment: String,
+}
+
+impl UserErrorType {
+    pub fn abi_type(&self) -> TypeRef {
+        TypeRef::Name(self.abi_type.clone())
+    }
+    pub fn typename(&self) -> TokenStream {
+        let t = &self.rich_type;
+        quote!(#t)
+    }
+    pub fn method_fragment(&self) -> &str {
+        &self.method_fragment
+    }
+}
diff --git a/crates/wiggle/generate/src/funcs.rs b/crates/wiggle/generate/src/funcs.rs
index 690ced5512..2c8cf2b350 100644
--- a/crates/wiggle/generate/src/funcs.rs
+++ b/crates/wiggle/generate/src/funcs.rs
@@ -1,6 +1,7 @@
 use proc_macro2::TokenStream;
 use quote::quote;
 
+use crate::error_transform::ErrorTransform;
 use crate::lifetimes::anon_lifetime;
 use crate::module_trait::passed_by_reference;
 use crate::names::Names;
@@ -9,6 +10,7 @@ pub fn define_func(
     names: &Names,
     func: &witx::InterfaceFunc,
     trait_name: TokenStream,
+    errxform: &ErrorTransform,
 ) -> TokenStream {
     let funcname = func.name.as_str();
 
@@ -43,16 +45,23 @@ pub fn define_func(
         quote!(())
     };
 
-    let err_type = coretype.ret.map(|ret| ret.param.tref);
-    let ret_err = err_type
-        .clone()
-        .map(|_res| {
+    let err_type = coretype.ret.clone().map(|ret| ret.param.tref);
+    let ret_err = coretype
+        .ret
+        .map(|ret| {
+            let name = ret.param.name.as_str();
+            let conversion = if let Some(user_err) = errxform.for_abi_error(&ret.param.tref) {
+                let method = names.user_error_conversion_method(&user_err);
+                quote!(#abi_ret::from(UserErrorConversion::#method(ctx, e)))
+            } else {
+                quote!(#abi_ret::from(e))
+            };
             quote! {
                 #[cfg(feature = "trace_log")]
                 {
-                    log::trace!("     | errno={}", e);
+                    log::trace!("     | {}={:?}", #name, e);
                 }
-                return #abi_ret::from(e);
+                return #conversion;
             }
         })
         .unwrap_or_else(|| quote!(()));
diff --git a/crates/wiggle/generate/src/lib.rs b/crates/wiggle/generate/src/lib.rs
index 699d33e216..bd6d5d9e40 100644
--- a/crates/wiggle/generate/src/lib.rs
+++ b/crates/wiggle/generate/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod config;
+mod error_transform;
 mod funcs;
 mod lifetimes;
 mod module_trait;
@@ -11,12 +12,13 @@ use quote::quote;
 use lifetimes::anon_lifetime;
 
 pub use config::Config;
+pub use error_transform::{ErrorTransform, UserErrorType};
 pub use funcs::define_func;
 pub use module_trait::define_module_trait;
 pub use names::Names;
 pub use types::define_datatype;
 
-pub fn generate(doc: &witx::Document, names: &Names) -> TokenStream {
+pub fn generate(doc: &witx::Document, names: &Names, errs: &ErrorTransform) -> TokenStream {
     // TODO at some point config should grow more ability to configure name
     // overrides.
     let rt = names.runtime_mod();
@@ -34,13 +36,24 @@ pub fn generate(doc: &witx::Document, names: &Names) -> TokenStream {
         }
     };
 
+    let user_error_methods = errs.iter().map(|errtype| {
+        let abi_typename = names.type_ref(&errtype.abi_type(), anon_lifetime());
+        let user_typename = errtype.typename();
+        let methodname = names.user_error_conversion_method(&errtype);
+        quote!(fn #methodname(&self, e: super::#user_typename) -> #abi_typename;)
+    });
+    let user_error_conversion = quote! {
+        pub trait UserErrorConversion {
+            #(#user_error_methods)*
+        }
+    };
     let modules = doc.modules().map(|module| {
         let modname = names.module(&module.name);
         let trait_name = names.trait_name(&module.name);
         let fs = module
             .funcs()
-            .map(|f| define_func(&names, &f, quote!(#trait_name)));
-        let modtrait = define_module_trait(&names, &module);
+            .map(|f| define_func(&names, &f, quote!(#trait_name), &errs));
+        let modtrait = define_module_trait(&names, &module, &errs);
         let ctx_type = names.ctx_type();
         quote!(
             pub mod #modname {
@@ -57,6 +70,7 @@ pub fn generate(doc: &witx::Document, names: &Names) -> TokenStream {
         pub mod types {
             #(#types)*
             #guest_error_conversion
+            #user_error_conversion
         }
         #(#modules)*
     )
diff --git a/crates/wiggle/generate/src/module_trait.rs b/crates/wiggle/generate/src/module_trait.rs
index b09f237028..c73cd052bb 100644
--- a/crates/wiggle/generate/src/module_trait.rs
+++ b/crates/wiggle/generate/src/module_trait.rs
@@ -1,6 +1,7 @@
 use proc_macro2::TokenStream;
 use quote::quote;
 
+use crate::error_transform::ErrorTransform;
 use crate::lifetimes::{anon_lifetime, LifetimeExt};
 use crate::names::Names;
 use witx::Module;
@@ -20,7 +21,7 @@ pub fn passed_by_reference(ty: &witx::Type) -> bool {
     }
 }
 
-pub fn define_module_trait(names: &Names, m: &Module) -> TokenStream {
+pub fn define_module_trait(names: &Names, m: &Module, errxform: &ErrorTransform) -> TokenStream {
     let traitname = names.trait_name(&m.name);
     let traitmethods = m.funcs().map(|f| {
         // Check if we're returning an entity anotated with a lifetime,
@@ -55,7 +56,14 @@ pub fn define_module_trait(names: &Names, m: &Module) -> TokenStream {
         let err = f
             .results
             .get(0)
-            .map(|err_result| names.type_ref(&err_result.tref, lifetime.clone()))
+            .map(|err_result| {
+                if let Some(custom_err) = errxform.for_abi_error(&err_result.tref) {
+                    let tn = custom_err.typename();
+                    quote!(super::#tn)
+                } else {
+                    names.type_ref(&err_result.tref, lifetime.clone())
+                }
+            })
             .unwrap_or(quote!(()));
 
         if is_anonymous {
diff --git a/crates/wiggle/generate/src/names.rs b/crates/wiggle/generate/src/names.rs
index 644b877d8c..47ab1312a6 100644
--- a/crates/wiggle/generate/src/names.rs
+++ b/crates/wiggle/generate/src/names.rs
@@ -3,7 +3,7 @@ use proc_macro2::{Ident, TokenStream};
 use quote::{format_ident, quote};
 use witx::{AtomType, BuiltinType, Id, Type, TypeRef};
 
-use crate::lifetimes::LifetimeExt;
+use crate::{lifetimes::LifetimeExt, UserErrorType};
 
 pub struct Names {
     ctx_type: Ident,
@@ -150,27 +150,45 @@ impl Names {
         format_ident!("{}_len", id.as_str().to_snake_case())
     }
 
-    pub fn guest_error_conversion_method(&self, tref: &TypeRef) -> Ident {
+    fn builtin_name(b: &BuiltinType) -> &'static str {
+        match b {
+            BuiltinType::String => "string",
+            BuiltinType::U8 => "u8",
+            BuiltinType::U16 => "u16",
+            BuiltinType::U32 => "u32",
+            BuiltinType::U64 => "u64",
+            BuiltinType::S8 => "i8",
+            BuiltinType::S16 => "i16",
+            BuiltinType::S32 => "i32",
+            BuiltinType::S64 => "i64",
+            BuiltinType::F32 => "f32",
+            BuiltinType::F64 => "f64",
+            BuiltinType::Char8 => "char8",
+            BuiltinType::USize => "usize",
+        }
+    }
+
+    fn snake_typename(tref: &TypeRef) -> String {
         match tref {
-            TypeRef::Name(nt) => format_ident!("into_{}", nt.name.as_str().to_snake_case()),
+            TypeRef::Name(nt) => nt.name.as_str().to_snake_case(),
             TypeRef::Value(ty) => match &**ty {
-                Type::Builtin(b) => match b {
-                    BuiltinType::String => unreachable!("error type must be atom"),
-                    BuiltinType::U8 => format_ident!("into_u8"),
-                    BuiltinType::U16 => format_ident!("into_u16"),
-                    BuiltinType::U32 => format_ident!("into_u32"),
-                    BuiltinType::U64 => format_ident!("into_u64"),
-                    BuiltinType::S8 => format_ident!("into_i8"),
-                    BuiltinType::S16 => format_ident!("into_i16"),
-                    BuiltinType::S32 => format_ident!("into_i32"),
-                    BuiltinType::S64 => format_ident!("into_i64"),
-                    BuiltinType::F32 => format_ident!("into_f32"),
-                    BuiltinType::F64 => format_ident!("into_f64"),
-                    BuiltinType::Char8 => format_ident!("into_char8"),
-                    BuiltinType::USize => format_ident!("into_usize"),
-                },
-                _ => panic!("unexpected anonymous error type: {:?}", ty),
+                Type::Builtin(b) => Self::builtin_name(&b).to_owned(),
+                _ => panic!("unexpected anonymous type: {:?}", ty),
             },
         }
     }
+
+    pub fn guest_error_conversion_method(&self, tref: &TypeRef) -> Ident {
+        let suffix = Self::snake_typename(tref);
+        format_ident!("into_{}", suffix)
+    }
+
+    pub fn user_error_conversion_method(&self, user_type: &UserErrorType) -> Ident {
+        let abi_type = Self::snake_typename(&user_type.abi_type());
+        format_ident!(
+            "{}_from_{}",
+            abi_type,
+            user_type.method_fragment().to_snake_case()
+        )
+    }
 }
diff --git a/crates/wiggle/macro/src/lib.rs b/crates/wiggle/macro/src/lib.rs
index 8fd6f680a7..214d14dd72 100644
--- a/crates/wiggle/macro/src/lib.rs
+++ b/crates/wiggle/macro/src/lib.rs
@@ -98,7 +98,10 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
     let doc = witx::load(&config.witx.paths).expect("loading witx");
     let names = wiggle_generate::Names::new(&config.ctx.name, quote!(wiggle));
 
-    let code = wiggle_generate::generate(&doc, &names);
+    let error_transform = wiggle_generate::ErrorTransform::new(&config.errors, &doc)
+        .expect("validating error transform");
+
+    let code = wiggle_generate::generate(&doc, &names, &error_transform);
     let metadata = if cfg!(feature = "wiggle_metadata") {
         wiggle_generate::generate_metadata(&doc, &names)
     } else {
diff --git a/crates/wiggle/tests/errors.rs b/crates/wiggle/tests/errors.rs
new file mode 100644
index 0000000000..a63199ddb9
--- /dev/null
+++ b/crates/wiggle/tests/errors.rs
@@ -0,0 +1,35 @@
+use wiggle_test::{impl_errno, WasiCtx};
+
+#[derive(Debug, thiserror::Error)]
+pub enum RichError {
+    #[error("Invalid argument: {0}")]
+    InvalidArg(String),
+    #[error("Won't cross picket line: {0}")]
+    PicketLine(String),
+}
+
+wiggle::from_witx!({
+    witx: ["tests/arrays.witx"],
+    ctx: WasiCtx,
+    errors: { errno => RichError },
+});
+
+impl_errno!(types::Errno, types::GuestErrorConversion);
+
+impl<'a> types::UserErrorConversion for WasiCtx<'a> {
+    fn errno_from_rich_error(&self, _e: RichError) -> types::Errno {
+        unimplemented!();
+    }
+}
+
+impl<'a> arrays::Arrays for WasiCtx<'a> {
+    fn reduce_excuses(
+        &self,
+        _excuses: &types::ConstExcuseArray,
+    ) -> Result<types::Excuse, RichError> {
+        unimplemented!()
+    }
+    fn populate_excuses(&self, _excuses: &types::ExcuseArray) -> Result<(), RichError> {
+        unimplemented!()
+    }
+}