This chapter considers only observational ecologic studies. Exploratory ecologic studies are a useful source of hypotheses about individual-level associations that can be followed up in studies that use individuals as the units of measurement and analysis. A narrow focus on individual-level associations may miss important spillover effects. Ecologic studies, in contrast, may detect them. Studies of the effectiveness of community-based intervention programs may involve determining whether presence of the law, policy, or program in a population is associated with a lower rate of adverse outcomes in that population. Because primary scientific interest focuses on group-level associations, ecologic studies are well matched to this purpose. The chapter discusses the design considerations in ecologic studies. It also focuses on concerns of special relevance to ecologic studies particularly when they are used for evaluating legal interventions, drawing freely on the Campbell and Stanley formulation.