Because of their well-documented advantages, randomized controlled trials (RCTs) represent the gold standard for testing hypotheses in medical research. However, RCTs are not ideal for addressing certain research questions ( e.g. , the risk for renal insufficiency associated with alcohol intake). In addition, conducting a properly designed RCT often requires substantial time and resources. For these reasons, including barriers of cost and need for rapid answers, the majority of clinical research studies in the renal literature use an observational design. Although there clearly is a great need for more RCTs that are conducted in populations with kidney disease, rigorous observational studies are extremely valuable and in many circumstances yield similar results as rigorous RCTs (1). Furthermore, although RCTs are considered the gold standard for examining the efficacy of a therapy, studies of prognosis often are addressed best by cohort studies. In this first of a multipart series dedicated to reviewing clinical research methods in nephrology, after providing a brief overview of observational studies, we focus on one of the major subtypes of observational designs: The cohort study. A glossary of common terms that are used in this and subsequent sections of this series is included in Table 1. Details on other methods ( e.g. , case-control studies, RCTs) will be the subject of future reviews. View this table: Table 1. Glossary The simplest form of observational study is the case report or case series, which describes the clinical course of individuals with a particular condition or diagnosis. Such studies often highlight a single clinical condition and at times even suggest a potential biologic mechanism. In a case series, the clinician gains an appreciation of the breadth of abnormalities ( e.g. , range of proteinuria in patients with HIV-associated nephropathy) that may characterize a single disease process. Although these studies may attempt to identify factors or treatments that …