Engagement in HIV care is increasingly recognized as a crucial step in maximizing individual patient outcomes. The recently updated HIV Medicine Association primary HIV care guidelines include a new recommendation highlighting the importance of extending adherence beyond antiretroviral medications to include adherence to clinical care. Beyond individual health, emphasis on a "test and treat" approach to HIV prevention highlights the public health importance of engagement in clinical care as an essential intermediary between the putative benefits of universal HIV testing ("test") followed by ubiquitous antiretroviral treatment ("treat"). One challenge to administrators, researchers and clinicians who want to systematically evaluate HIV clinical engagement is deciding on how to measure retention in care. Measuring retention is complex as this process includes multiple clinic visits (repeated measures) occurring longitudinally over time. This article provides a synthesis of five commonly used measures of retention in HIV care, highlighting their methodological and conceptual strengths and limitations, and suggesting situations where certain measures may be preferred over others. The five measures are missed visits, appointment adherence, visit constancy, gaps in care, and the Human Resources and Services Administration HIV/AIDS Bureau (HRSA HAB) performance measure for retention in HIV care. As has been noted for antiretroviral medication adherence, there is no gold standard to measure retention in care, and consideration of the advantages and limitations of each measure, particularly in the context of the desired application, should guide selection of a retention measure. Copyright © 2010, Mary Ann Liebert, Inc.