This article analyzes the relative importance of falling electoral loss rates and retirement rates in creating the structure of tenure in the modern Congress. We develop a dynamic model in which electoral loss and voluntary retirement determine a unique long-run equilibrium (or steady-state) tenure profile, which is the fraction of members in each term of service. The model is then used to derive steady-state tenure profiles for the period 1870 to 1930 and to compare them to profiles that are simulated under different counterfactual hypotheses about loss and retirement. We briefly consider more recent congresses as well. We find that declining retirement accounts for 75% of the total long-run change in turnover from 1870 to 1930. However, declining electoral loss accounts for 82% of the total long-run change in turnover from the period of “pre-incumbency” (1926–1946) to the 1980s (1982–1988).