Biomedical research may be experimental or observational, depending on whether the investigator assigned the exposure. Experimental studies, where the investigator assigns the intervention, can be conducted as either randomized or non-randomized study. Observational studies can be purely descriptive or analytical. In the latter case, they feature a control group for comparison. Cohort studies are analytical observational studies that track subjects forward in time from exposure to outcome—they are thus prospective in nature. In contrast, case-control studies work retrospectively, tracking back from outcome to exposure, through review of medical records and history. Cross-sectional studies assess for both exposure and outcome simultaneously. Descriptive studies which do not have a control group, may deal with individuals or populations. Case reports and case series deal with individuals while surveillance deals with populations. Aggregate-level studies include ecological correlational studies. A clinical trial is an interventional research study that prospectively assigns human participants to health-related interventions. Clinical trials are conducted in phases from I to IV. All study designs have their place to answer different types of research questions. Retrospective case series are useful for defining natural history. Cross-sectional studies are the best way to determine prevalence. Cross-sectional studies may also be appropriate for an initial evaluation of the accuracy of new diagnostic tests as compared to a gold standard. Case-control studies are used to elucidate potential etiologies of rare diseases especially those with long latent period between an exposure and the disease. Cohort studies can be used to assess natural history/prognosis or to assess potential disease etiologies. Randomized controlled trials are designed to assess the efficacy of therapeutic interventions.