Aspect Oriented Programming (AOP) is a new programming paradigm being researched upon at present. AOP proposes a number of new ideas and methods to efficiently deal with software complexity in arenas where the Object-Oriented Paradigm fails to provide even a suboptimal solution. In this thesis, we present AOPInjector, which is a post-compilation aspect oriented programming tool designed for the Microsoft .NET framework. It is an attempt to understand the underlying concepts of AOP as well as a proof of concept of an extensible tool to provide AOP support for applications developed with the .NET framework. The following design goals were considered: To keep the syntax as hygienic as possible and consistent with the syntax of the C# programming language; To reduce the learning curve of a AOP programmer using the tool; Finally to rely upon efficient IL code injection at the post compilation phase instead of run time method interceptions. The implementation of the tool in this thesis has served as a realization of these goals.