The two aims of this paper are (i) to identify the factors influencing the decision to drop out of high school and (ii) to estimate how the risk of dropping out varies during a student s high school years. The study is based on data from the National Educational Longitudinal Study 1988-94 and uses event history techniques to investigate the dropout decision. A flexible piecewise-linear baseline hazard is estimated, controlling for unobserved heterogeneity both parametrically and non-parametrically. Factors such as previous educational attainment, ethnicity and a range of family background variables are found to influence the dropout decision. The risk of dropping out is estimated to vary substantially during the student s time in secondary school.