© Article author(s) (or their employer(s) unless otherwise stated in the text of the article) 2018. All rights reserved. No commercial use is permitted unless otherwise expressly granted. Background Health systems are currently subject to unprecedented financial strains. Inappropriate test use wastes finite health resources (overuse) and delays diagnoses and treatment (underuse). As most patient care is provided in primary care, it represents an ideal setting to mitigate waste. Objective To identify overuse and underuse of diagnostic tests in primary care. Design Systematic review and meta-analysis. Data sources and eligibility criteria We searched MEDLINE and Embase from January 1999 to October 2017 for studies that measured the inappropriateness of any diagnostic test (measured against a national or international guideline) ordered for adult patients in primary care. Results We included 357 171 patients from 63 studies in 15 countries. We extracted 103 measures of inappropriateness (41 underuse and 62 overuse) from included studies for 47 different diagnostic tests. The overall rate of inappropriate diagnostic test ordering varied substantially (0.2%-100%)%). 17 tests were underused >50% of the time. Of these, echocardiography (n=4 measures) was consistently underused (between 54% and 89%, n=4). There was large variation in the rate of inappropriate underuse of pulmonary function tests (38%-78%, n=8). Eleven tests were inappropriately overused >50% of the time. Echocardiography was consistently overused (77%-92%), whereas inappropriate overuse of urinary cultures, upper endoscopy and colonoscopy varied widely, from 36% to 77% (n=3), 10%-54% (n=10) and 8%-52% (n=2), respectively. Conclusions There is marked variation in the appropriate use of diagnostic tests in primary care. Specifically, the use of echocardiography (both underuse and overuse) is consistently poor. There is substantial variation in the rate of inappropriate underuse of pulmonary function tests and the overuse of upper endoscopy, urinary cultures and colonoscopy. PROSPERO registration number CRD42016048832.