It has been said that everything is a catalyst for something. Although profound, the statement is not very useful unless materials are organized into groups with common explained with theories or models, and systematized into patterns from which new catalysts may be predicted. In this chapter, we examine common types of catalytic materials, current theories underlining their mode of action, and activity patterns useful in design. Much of this is brief by necessity, but the interested reader will find sufficient references for further study. For the casual reader, this chapter illustrates the complex background in catalysis and testifies to the current attempts to lift catalysis from an "art" to a "science."