The northern Indian Ocean undergoes a major transition from anthropogenic aerosols during the northeasterly winter monsoon season to mineral dust and sea salt during the southwest summer monsoon. The former is dominated by low-level transport from south and southeast Asia, while the latter results from low to mid tropospheric transport from the African continent and the Arabian Peninsula. During the winter monsoon, low-level transport from the Indian subcontinent and neighboring nations is particularly important. This paper uses 5 years of satellite-derived aerosol optical depths (AODs) to document the large seasonal variations in AODs modulated by the monsoons. In situ data are used to develop and validate the procedure we employ to derive visible AODs from the radiances observed by the advanced very high resolution radiometer (AVHRR) onboard polar-orbiting satellites. Monthly mean AOD over the Arabian Sea displays a clear annual cycle with a maximum (0.60 ¿ 0.10) in July and a minimum (0.20 ¿ 0.05) in January. The summer monsoon maximum is due both to the southwest winds bringing dust from the Horn of Africa over the Arabian Sea and mid tropospheric transport of dust from the Arabian Peninsula. Such long-range transports also lead to aerosol loading south of the equator. We also show that the Indonesia forest fires during the 1997 El Ni¿o led to a large increase in AOD over most of the equatorial Indian Ocean.