We use Bayesian meta-analysis methods to estimate the impact of unconditional cash transfers (UCTs). Aggregating evidence from 115 studies of 72 UCT programs in middle and low income countries, we find strong and positive average treatment effects on ten of thirteen outcomes: monthly household total and food consumption, monthly income, labor supply, school enrollment, food security, psychological well-being, total assets, financial assets, and children height-for-age. We examine seven specific theoretical and policy hypotheses, such as presence of savings frictions, dynamic effects, curvature of marginal returns, targeting effects, “nudge” effects, labor supply elasticity and related “dependency” theories, and contextual heterogeneity.