Ecological studies provide important and frequent sources of evidence of environmental health, since their unit of analysis is populations. This review summarizes the foundations of ecological studies with the premise that they can be performed using quantitative, qualitative or mixed methods. It presents the logic behind their design, their role in exploring causality, the variables and categories of analysis and the design principles and techniques used to collect data. Examples of ecological studies performed in Latin America are then presented, as well as some common methodological problems and options to address them. Lastly, the relevance of quantitative and qualitative ecological studies to environmental health as a way to overcome the dominance of conceptual and methodological individualism is highlighted, though ecological studies alone do not suffice for studying population health.