How much postsecondary education costs families, and how much is publicly financed, varies immensely across countries and the proper balance is hotly debated. The United States, despite having a highly privately financed system, is home to hundreds of local and provincial (i.e. state) 'free college' programmes. I review the growing literature on these programmes, discussing postulated causal mechanisms through which they may work and summarizing effects on students, communities and colleges, with an emphasis on causal evidence. Most proposed mechanisms lack empirical support or are implausible for most existing programmes. Programmes are consistently found to change postsecondary destinations and increase enrolment at eligible colleges. Less consistently, they boost postsecondary participation and gross educational attainment, while evidence for positive effects on postsecondary performance and attainment net of participation is mixed. There is insufficient or inconsistent evidence for effects on secondary school performance, post-college income, or inequality reduction according to gender, race, or socioeconomic status.