In the world of AI, the expert system is one of the oldest--and richest--topics of discussion. Not entirely a system of "machine learning", the expert system still represents a powerful tool for giving users the "right" answer when the solution is a complex sequence of forward- and back-tracking permutations, such as determining the best wine to choose with dinner, how to win the classic river-crossing puzzles, or schedule employees in the face of their various constraints and time-off requests. In this session, we'll talk about what an expert system is, what a RETE engine is, and how they can be used in a mainstream environment like .NET.