Southeast Asia is a region where the number of people infected with HIV/AIDS is one of the fastest growing in the world. Tuberculosis (TB) has grown along with the HIV epidemic. TB is not only the most common AIDS-defining illness but is also the leading cause of morbidity and mortality in AIDS patients. Cryptococcosis (meningitis or disseminated) is one of the most common opportunistic infections in AIDS patients. Cryptococcal meningitis is the first in the differential diagnosis considered with meningeal irritation. Penicillosis, a unique systemic mycosis, is an important emerging public health problem and has been classified as an AIDS defining illness in endemic areas like Thailand. Pneumocystis carinii (jiroveci) pneumonia has been one of the most important opportunistic infections in AIDS patients. Among parasitic infections, cryptosporidiosis is the most common intestinal protozoan infection relating to diarrhea in AIDS patients and toxoplasmosis is the only parasitic infection of the nervous system with a substantial incidence, up to 14.8%. Cytomegalovirus (CMV) retinitis has a lower prevalence compared to other opportunistic infections. In the era of highly active antiretroviral therapy (HAART), the incidence of opportunistic infections has significantly reduced in the past few years. Subsequently, the phenomena of immune restoration inflammatory syndrome (IRIS) in AIDS patients has been reported in this region as a result of HAART.