Evidence is accumulating that rates of molecular evolution vary substantially between species, and that this rate variation is partly determined by species characteristics. A better understanding of how and why rates of molecular evolution vary provides a window on evolutionary processes, and might facilitate improvements in DNA sequence analysis. Measuring rates of molecular evolution and identifying the correlates of rate variation present a unique set of challenges. We describe and compare recent methodological advances that have been proposed to deal with these challenges. We provide a guide to the theoretical basis and practical application of the methods, outline the types of data on which they can be used, and indicate the types of questions they can be used to ask.