In this paper we present an approach towards safe software composition based on aspect-orientation. Aspects enable the systematic addition of code into existing programs but often they also introduce errors. In order to provide safe aspects for software composition we address the verification of the aspect-oriented language paradigm. We construct a basic calculus for aspects with types and prove formally type safety. More precisely, this paper presents the following contributions (a) a fully formalized type system for the Theory of Objects including the proof of type safety, (b) a theory of aspects based on the Theory of Objects including a type system for aspects, and (c) the definition of a notion of type safety for aspects including its proof. The entire theory and proofs are carried out in the theorem prover Isabelle/HOL
