Why do women engage in transactional sex? While much of the explanation is that sex-for-money pays more than other jobs, we use a unique panel data set constructed from 192 self-reported diaries of sex workers in Western Kenya to show that women who supply transactional sex develop relationships with regular clients, and that these clients send transfers in response to negative income shocks. Regular clients are the primary source of interperson insurance that women receive, and women report in a separate survey that client transfers are an important reason that they participate in the market. (Contains 10 tables and 37 footnotes.)