Although Highly Active Anti-Retroviral Therapy (HAART) significantly reduced HIV/AIDS mortality, appropriate adherence level is recommended for viral suppression and therapeutic response in People Living with HIV/AIDS (PLWHA). In the most studies, adherence is defined as taking ≥95% of prescribed medications. Poor or non-adherence may lead to treatment failure and drug resistance. There is no golden standard for evaluation of adherence to medication and many measurement methods are used to assess adherence rate. Moreover, several determinants have been contemplated for adherence in different studies; however, the exact roles of some determinants are not well established. The goals of this review are to describe the adherence rates, to discuss the advantages and disadvantages of common adherence measurement methods, to examine significant correlations related to adherence and to recommend strategies for improving adherence in clinical care.