Since its introduction, the proportional hazards model proposed by Cox has become the workhorse of regression analysis for censored data. In the last several years, the theoretical basis for the model has been solidified by connecting it to the study of counting processes and martingale theory. These developments have, in turn, led to the introduction of several new extensions to the original model. These include the analysis of residuals, time dependent coefficients, multiple/correlated observations, multiple time scales and time dependent strata.