Retention in care and adherence to antiretroviral treatment (ART) are critical elements of HIV care interventions and are closely associated with optimal individual and public health outcomes and cost effectiveness. This literature review was conducted to analyse how the roles of clients in HIV care and treatment are discussed, from terminology used to measurement methods to consequences of a wide range of patient-related factors impacting client adherence to ART and retention in care. Unfortunately, data suggests that clients find it hard to follow recommended behaviour. For HIV, the greatest loss to follow-up occurs before starting treatment, though each step of the continuum of care is affected. Measurement approaches can be divided into ‘direct’ and ‘indirect’ methods; in practice, a combination is often considered the best strategy. Inadequate retention and adherence lead to decreased health outcomes (morbidity, mortality, drug resistances, risk of transmission) and cost effectiveness (increased costs and lower productivity).