 Freedman's main theorem about Casson handles states that they are all homeomorphic to formula_4; or in other words they are topological 2-handles.In general they are not diffeomorphic to formula_4 as follows from Donaldson's theorem, and there are an uncountable infinite number of different diffeomorphism types of Casson handles.However the interior of a Casson handle is diffeomorphic to formula_8; Casson handles differ from standard 2 handles only in the way the boundary is attached to the interior.