In this theoretical article, we review central concepts in the psychological literature on relationship commitment to provide a foundation to discuss two themes related to long-term romantic relationships and marriages. First, we describe and discuss the role that commitment plays in stabilizing romantic attachment. Second, we use empirical research on cohabitation to highlight how what are now common trajectories of couple development can undermine the formation of commitment. The first topic underscores an increasingly important role for commitment in an age of companionate marriage. The second topic draws attention to dynamics that can affect the strength of romantic commitments, especially in marriage.