Rumination has been widely studied and is a crucial component in the study of cognitive vulnerabilities to depression. However, rumination means different things in the context of different theories, and has not been uniformly defined or measured. This article aims to review models of rumination, as well as the various ways in which it is assessed. The models are compared and contrasted with respect to several important dimensions of rumination. Guidelines to consider in the selection of a model and measure of rumination are presented, and suggestions for the conceptualization of rumination are offered. In addition, rumination's relation to other similar constructs is evaluated. Finally, future directions for the study of ruminative phenomena are presented. It is hoped that this article will be a useful guide to those interested in studying the multi-faceted construct of rumination.