Epidemiology literally means "the study of what is upon the people." It puts the individual's condition in a population context and is the path to disease prevention. In the first part of this review, important aspects of epidemiology are discussed. Fundamentals of epidemiologic research include the measurement of occurrence of an event (prevalence and incidence) and the identification of factors that are associated with this event. The main study designs in observational studies are cohort, case-control, and cross-sectional studies, all of which have intrinsic strengths and limitations. These limitations include a variety of biases, which can be regrouped into selection bias, information bias, and confounding. The STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) checklist is an important tool to further improve the reporting and quality of epidemiologic studies, and it is introduced. In the second part of this review, practical examples are presented, illustrating how dermatoepidemiology has contributed to an improved understanding of skin diseases and patient care, specifically in the case of melanoma therapy, serious cutaneous adverse reactions, Lyme disease, long-term safety of psoralin plus UVA (PUVA), teratogenicity of isotretinoin, and comorbidities in psoriasis.