The book provides an introduction to the principles and methods of epidemiology, with application both to public health and clinical practice (clinical epidemiology). These principles and strategies are also applied to critical appraisal, providing tools that will help students and professionals evaluate the quality of research studies. This third edition adds more recent examples, including to the COVID-19 pandemic. Other updates include the following: 1. Links to Statistics Canada resources have been updated, reflecting discontinuation of the CANSIM system. 2. Sections on systematic reviews and meta-analyses have been updated, along with expanded reference to relevant EQUATOR reporting standards and checklists. 3. The coverage of causal diagrams has been expanded, with an emphasis on directed acyclic graphs (DAGs). The discussion of causation has been expanded to include counterfactual approaches to questions of causality. 4. Many of the examples have been updated with reference to recent Canadian and international literature.