The purpose of this clinical practice update is to define key modalities in the diagnosis and monitoring of celiac disease (CD) in adults as well as in children and adolescents. Methods: The recommendations outlined in this expert review are based on available published evidence, including cohort and case-control studies of the diagnostic process as well as controlled and descriptive studies of disease management. Best Practice Advice 1: Serology is a crucial component of the detection and diagnosis of CD, particularly tissue transglutaminase-IgA (TG2-IgA), IgA testing, and less frequently endomysial IgA testing. Best Practice Advice 2: Thorough histological analysis of duodenal biopsies with Marsh classification, counting of lymphocytes per HPF, and morphometry is important for diagnosis as well as for differential diagnosis. Best Practice Advice 2a: TG2-IgA, at high levels (> x 10 upper normal limit) is a reliable and accurate test for diagnosing active CD. When such a strongly positive TG2-IgA is combined with a positive endomysial antibody in a second blood sample, the positive predictive value for CD is virtually 100%. In adults, esophagogastroduodenoscopy (EGD) and duodenal biopsies may then be performed for purposes of differential diagnosis. Best Practice Advice 3: IgA deficiency is an infrequent but important explanation for why patients with CD may be negative on IgA isotype testing despite strong suspicion. Measuring total IgA levels, IgG deamidated gliadin antibody tests, and TG2-IgG testing in that circumstance is recommended. Best Practice Advice 4: IgG isotype testing for TG2 antibody is not specific in the absence of IgA deficiency.