Complexity factors were introduced by Wilbur Nelson in the 1960s to compare two refineries relative to their sophistication and capital intensity. The idea proved useful and has been widely adopted and applied throughout the industry. The chapter begins by defining an ideal refinery and introducing complexity factors, complexity cross factors, and methodologies used in their assessment. Complexity indices differentiate refinery type and are described along with a snapshot of U.S. and world refining statistics circa 2018. The refinery complexity equation is used to quantify changes in the complexity index for changes in process capacities and refinery configuration. By using cost functions, a more precise and transparent formulation of complexity factor emerges, and the chapter concludes with a functional formulation of complexity factor. Some of the limitations of refinery complexity are also described and illustrated.