Mixin modules are proposed as an extension of a class-based programming language. Mixin modules combine parallel extension of classes, including extension of the self types for those classes, with mixin-based inheritance. For soundness of sybtyping purposes, they require an explicit distinction between mixin-based objects and class-based objects. Applications of mixin modules are in statically type-safe monad-based aspect-oriented programming, and in modular mixin-based Internet programming.