diff --git a/src/gen.rs b/src/gen.rs
index 2729624..0b7fc92 100644
--- a/src/gen.rs
+++ b/src/gen.rs
@@ -2,8 +2,9 @@ use crate::proc_macro::Span;
 use proc_macro2::TokenStream as TokenStream2;
 use quote::{ToTokens, TokenStreamExt};
 use syn::{
-    FnArg, Ident, ItemTrait, Lifetime, MethodSig, Pat, PatIdent, TraitItem, TraitItemConst,
-    TraitItemMethod, TraitItemType,
+    FnArg, Ident, ItemTrait, Lifetime, MethodSig, Pat, PatIdent, ReturnType, TraitBound,
+    TraitBoundModifier, TraitItem, TraitItemConst, TraitItemMethod, TraitItemType, Type,
+    TypeParamBound, WherePredicate,
 };
 
 use crate::{
@@ -65,12 +66,84 @@ fn gen_header(
     // The `'{proxy_lt_param}` in the beginning is only added when the proxy
     // type is `&` or `&mut`.
     let impl_generics = {
+        // Determine whether we can add a `?Sized` relaxation to allow trait
+        // objects. We can do that as long as there is no method that has a
+        // `self` by value receiver and no `where Self: Sized` bound.
+        let sized_required = trait_def.items.iter()
+            // Only interested in methods
+            .filter_map(|item| if let TraitItem::Method(m) = item { Some(m) } else { None })
+            // We also ignore methods that we will not override. In the case of
+            // invalid attributes it is save to assume default behavior.
+            .filter(|m| !should_keep_default_for(m, proxy_type).unwrap_or(false))
+            .any(|m| {
+                // Check if there is a `Self: Sized` bound on the method.
+                let self_is_bounded_sized = m.sig.decl.generics.where_clause.iter()
+                    .flat_map(|wc| &wc.predicates)
+                    .filter_map(|pred| {
+                        if let WherePredicate::Type(p) = pred { Some(p) } else { None }
+                    })
+                    .any(|pred| {
+                        // Check if the type is `Self`
+                        match &pred.bounded_ty {
+                            Type::Path(p) if p.path.is_ident("Self") => {
+                                // Check if the bound contains `Sized`
+                                pred.bounds.iter().any(|b| {
+                                    match b {
+                                        TypeParamBound::Trait(TraitBound {
+                                            modifier: TraitBoundModifier::None,
+                                            path,
+                                            ..
+                                        }) => path.is_ident("Sized"),
+                                        _ => false,
+                                    }
+                                })
+                            }
+                            _ => false,
+                        }
+                    });
+
+                // Check if the first parameter is `self` by value. In that
+                // case, we might require `Self` to be `Sized`.
+                let self_value_param = match m.sig.decl.inputs.first().map(|p| p.into_value()) {
+                    Some(FnArg::SelfValue(_)) => true,
+                    _ => false,
+                };
+
+                // Check if return type is `Self`
+                let self_value_return = match &m.sig.decl.output {
+                    ReturnType::Type(_, t) => {
+                        if let Type::Path(p) = &**t {
+                            p.path.is_ident("Self")
+                        } else {
+                            false
+                        }
+                    }
+                    _ => false,
+                };
+
+                // TODO: check for `Self` parameter in any other argument.
+
+                // If for this method, `Self` is used in a position that
+                // requires `Self: Sized` or this bound is added explicitly, we
+                // cannot add the `?Sized` relaxation to the impl body.
+                self_value_param || self_value_return || self_is_bounded_sized
+            });
+
+        let relaxation = if sized_required {
+            quote! {}
+        } else {
+            quote! { + ?::std::marker::Sized }
+        };
+
         // Determine if our proxy type needs a lifetime parameter
         let (mut params, ty_bounds) = match proxy_type {
-            ProxyType::Ref | ProxyType::RefMut => {
-                (quote! { #proxy_lt_param, }, quote! { : #proxy_lt_param + #trait_path })
+            ProxyType::Ref | ProxyType::RefMut => (
+                quote! { #proxy_lt_param, },
+                quote! { : #proxy_lt_param + #trait_path #relaxation }
+            ),
+            ProxyType::Box | ProxyType::Rc | ProxyType::Arc => {
+                (quote!{}, quote! { : #trait_path #relaxation })
             }
-            ProxyType::Box | ProxyType::Rc | ProxyType::Arc => (quote!{}, quote! { : #trait_path }),
             ProxyType::Fn | ProxyType::FnMut | ProxyType::FnOnce => {
                 let fn_bound = gen_fn_type_for_trait(proxy_type, trait_def)?;
                 (quote!{}, quote! { : #fn_bound })
diff --git a/tests/compile-fail/trait_obj_value_self.rs b/tests/compile-fail/trait_obj_value_self.rs
new file mode 100644
index 0000000..2e818d0
--- /dev/null
+++ b/tests/compile-fail/trait_obj_value_self.rs
@@ -0,0 +1,13 @@
+use auto_impl::auto_impl;
+
+
+#[auto_impl(Box)]
+trait Trait {
+    fn foo(self);
+}
+
+fn assert_impl<T: Trait>() {}
+
+fn main() {
+    assert_impl::<Box<dyn Trait>>();
+}
diff --git a/tests/compile-pass/trait_obj_default_method.rs b/tests/compile-pass/trait_obj_default_method.rs
new file mode 100644
index 0000000..9ee6962
--- /dev/null
+++ b/tests/compile-pass/trait_obj_default_method.rs
@@ -0,0 +1,22 @@
+use auto_impl::auto_impl;
+
+
+#[auto_impl(Box)]
+trait Trait {
+    fn bar(&self);
+
+    #[auto_impl(keep_default_for(Box))]
+    fn foo(self) where Self: Sized {}
+}
+
+fn assert_impl<T: Trait>() {}
+
+struct Foo {}
+impl Trait for Foo {
+    fn bar(&self) {}
+}
+
+fn main() {
+    assert_impl::<Foo>();
+    assert_impl::<Box<dyn Trait>>();
+}
diff --git a/tests/compile-pass/trait_obj_immutable_self.rs b/tests/compile-pass/trait_obj_immutable_self.rs
new file mode 100644
index 0000000..1ac30c0
--- /dev/null
+++ b/tests/compile-pass/trait_obj_immutable_self.rs
@@ -0,0 +1,19 @@
+use auto_impl::auto_impl;
+
+
+#[auto_impl(&, &mut, Box, Rc, Arc)]
+trait Trait {
+    fn foo(&self);
+}
+
+fn assert_impl<T: Trait>() {}
+
+fn main() {
+    use std::{rc::Rc, sync::Arc};
+
+    assert_impl::<&dyn Trait>();
+    assert_impl::<&mut dyn Trait>();
+    assert_impl::<Box<dyn Trait>>();
+    assert_impl::<Rc<dyn Trait>>();
+    assert_impl::<Arc<dyn Trait>>();
+}
diff --git a/tests/compile-pass/trait_obj_value_self.rs b/tests/compile-pass/trait_obj_value_self.rs
new file mode 100644
index 0000000..ebf30eb
--- /dev/null
+++ b/tests/compile-pass/trait_obj_value_self.rs
@@ -0,0 +1,7 @@
+use auto_impl::auto_impl;
+
+
+#[auto_impl(Box)]
+trait Trait {
+    fn foo(self);
+}