Abstract In the last decade, studies in persons with HIV (PWH) on antiretroviral therapy (ART) have shed light on the significance of persistently high CD8 counts and low CD4/CD8 ratios. A low CD4/CD8 ratio reflects increased immune activation and is associated with an increased risk of severe non-AIDS events. As a result, many clinicians now believe that the CD4/CD8 ratio can help in HIV monitoring, and many researchers now report it as an efficacy marker in interventional studies. However, the topic is more complex. Recent studies have not yielded unanimous conclusions on the ability of the CD4/CD8 ratio to predict adverse outcomes, and only some clinical guidelines recommend monitoring it. Knowledge gaps remain on the best cutoff points, associated clinical events, effects of treatments, and how the CD4/CD8 ratio could improve decision making in the clinic. Here, we critically review the literature, identify knowledge gaps, and discuss the role of the CD4/CD8 ratio as a marker for HIV monitoring.