Advances in quantum chemical methods in combination with exponential growth in the computational speed of computers have enabled researchers in the field of catalysis to apply electronic structure calculations to a wide variety of increasingly complex problems. Such calculations provide insights into why and how changes in the composition and structure of catalytically active sites affect their activity and selectivity for targeted reactions. The aim of this review is to survey the recent advances in the methods used to make quantum chemical calculations and to define transition states as well as to illustrate the application of these methods to a selected series of examples taken from the authors' recent work.