This book will introduce you to research designs and methods in global health. Global health is a multi-disciplinary field, so we consider approaches common to the behavioral and social sciences, public health, and medicine. The primary focus is causal inference. In global health, we are often interested in knowing what programs, policies, and interventions "work" and why. To answer questions of impact, we often turn to randomized controlled trials, a mainstay of medical research. As such, we review the rationale, process, and limitations of field experiments. Randomization is not always possible or advisable, however, and researchers must build a causal argument using non-experimental methods. Therefore we also review several quasi-experimental and observational designs, consider relevant threats to causal inference, and discuss how to improve designs to reduce threats to internal validity. As we build up to this discussion, we cover research basics, such as developing and testing theory, asking good questions, understanding variability, designing good measurement, and selecting research participants.