Why do couples cohabit before getting married? To answer this, I exploit a survival pension reform in Sweden that imposed eligibility deadline, effectively penalising female partners who married after a cutoff date. The empirical evidence shows that the reform primarily induced long-term cohabiters—those who likely would not have married in the absence of the reform—to marry before the deadline, while newer cohabiters were largely unaffected. To interpret these patterns, I develop a structural life-cycle model in which couples learn about match quality, bargain over their relationship, and choose between cohabitation, marriage, or being single. In the model, cohabitation and marriage function both as complements and substitutes. In the early stages of a relationship, cohabitation complements marriage by allowing couples to gather information about their compatibility before getting married. For long-term cohabiters, however, cohabitation increasingly serves as a substitute, providing many of the same benefits without the formal commitment. The model matches the empirical facts well. I use the model to assess the value of information: couples are willing to give up almost 7% of consumption in each period in order to retain learning. Counterfactual experiments show that de facto marriage laws remove much of the information advantage of cohabitation, leading to earlier but less selective marriages. Joint taxation increases the number of marriages and overall welfare through fiscal transfers, but reduces marital stability.