Marriage data show a strong degree of positive assortative mating along a variety of attributes. But since marriage is an equilibrium outcome, it is unclear whether positive sorting is the result of preferences rather than opportunities. We assess the relative importance of preferences and opportunities in dating behaviour, using unique data from a large commercial speed dating agency. While the speed dating design gives us a direct observation of individual preferences, the random allocation of participants across events generates an exogenous source of variation in opportunities and allows us to identify the role of opportunities separately from that of preferences. We find that both women and men equally value physical attributes, such as age and weight, and that there is positive sorting along age, height, and education. The role of individual preferences, however, is outplayed by that of opportunities. Along some attributes (such as occupation, height and smoking) opportunities explain almost all the estimated variation in demand. Along other attributes (such as age), the role of preferences is more substantial, but never dominant. Despite this, preferences have a part when we observe a match, i.e., when two individuals propose to one another.