Immune reconstitution inflammatory syndrome (IRIS) is a phenomenon observed in patients recovering from immunodeficiency. The clinical presentation of IRIS involves the unmasking of covert infections or the worsening of overt conditions. Several causes and pathways have been suggested, most recognizing an inflammatory flare component occurring in the context of rapid immune reconstitution. In HIV-infected patients, IRIS inadvertently occurs as the consequence of successful antiretroviral therapy, and it is affiliated with improvement of the immune function, complicating the course of the disease and presenting treatment challenges to clinicians. The pathogenesis of IRIS is poorly understood, but in recovering HIV patients, its initiation and progression seem to be primarily linked to an increase in CD4+ T-helper and CD8+ T-suppressor cell count and a reduction in T-regulatory cells, all endorsed by exaggerated cytokine release and activity. The clinical presentation of IRIS is usually atypical. The manifestations depend on the trigger antigen, which can be an infective agent (viable or nonviable), a host antigen, or a tumor antigen. Most IRIS cases are self-limiting, but a few cases can be overwhelming and life-threatening; hence, early recognition is important. In most cases, there is no need to discontinue the antiretroviral therapy, although in the more severe cases, other clinical intervention may be necessary.