Western countries differ greatly in the extent to which single mothers participate in the labor market. Using LIS data for 15 countries, I propose and estimate a simple structural model of labor supply that incorporates the main variables that influence the work decision for single mothers. The results suggest that a large part of the cross country variation in the employment rates of single mothers can be explained by their different demographic characteristics and by the variation in expected income in the in-work versus out-of-work states. Women with higher expected earnings are more likely to work. Higher in-work benefits encourage employment. Single mothers with higher income from other sources, including child support, are less likely to work. Even after demographic and income variables are controlled for, the country dummies remain significant. This indicates that other variables not explicitly incorporated in the model, such as childcare arrangements or social and cultural backgrounds, may also play a relevant role.