Kaposi sarcoma (KS) is a low-grade vascular tumor that typically manifests as one of four variants: classic KS, endemic (African) KS, iatrogenic (organ transplant-related) KS, or acquired immunodeficiency syndrome (AIDS)-related KS. Several clinical and epidemiologic differences have been noted among these variants. Classic KS and endemic KS rarely require radiologic evaluation due to their usually chronic course and stability of skin compromise. However, iatrogenic KS and AIDS-related KS, the most common forms of the disease, are frequently disseminated or symptomatic and may thus require imaging studies for both diagnosis and staging. KS is the most common tumor among AIDS patients, affecting a high percentage of these individuals, and is considered to be an AIDS-defining illness. Multiple organs can be involved by AIDS-related KS. KS has been linked with human herpes virus type 8 infection and other cofactors. Although pulmonary, gastrointestinal, and skin involvement by KS has previously been described, this tumor can affect multiple organs, generating a wide spectrum of imaging findings and pathologic correlates. It is important for the radiologist to be familiar with this spectrum of imaging manifestations and corresponding pathologic findings. Copyright RSNA, 2006