Following up test results in a timely fashion is a recognized patient safety problem in primary care, and inadequate follow-up systems are a source of frustration for outpatient clinicians and a relatively common source of malpractice claims. This systematic review found evidence that failure to act on abnormal radiology or laboratory results is common and clearly linked to missed or delayed diagnoses. The review also found wide variation in processes for handling test results across studies. Electronic health records (EHRs) did appear to improve test follow-up rates, although a substantial proportion of abnormal results were not followed up even with EHRs. The authors advocate for more standardized processes for informing patients of abnormal results, and recent guidelines have been published for organizational policies to improve test result communication.