Analyses of two-particle correlations have provided the chief means for determining spatio-temporal characteristics of relativistic heavy ion collisions. We discuss the theoretical formalism behind these studies and the experimental methods used in carrying them out. Recent results from RHIC are put into context in a systematic review of correlation measurements performed over the past two decades. The current understanding of these results is discussed in terms of model comparisons and overall trends. * NB: if you like that, this would be based on my recent review article with Urs, Scott, and Ron, written up in Ann. Rev. Nucl. Part. Sci. 55 311 (2005); nucl-ex/0505014