Opisthorchis viverrini is a human liver fluke endemic in Thailand, Lao PDR and Cambodia. Three subspecies of Bithynia siamensis and 18 species of cyprinoid fish are susceptible first and second intermediate hosts, respectively. The differentiation between the adult worm of O. viverrini and that of the closely related species, Opisthorchis felineus and Clonorchis sinensis, is mainly based on the shape and position of the testes and the arrangement of the vitellaria. Eggs of these flukes are morphologically similar and problematic in identification. However, the differentiation between opisthorchid and lecithodendrid eggs is possible. Morphology and biology of each stage are comparatively reviewed. Biological aspects of snail and fish hosts are also included.