The discovery history of opisthorchiid flukes dates back more than 200 years. The life cycle of these liver flukes includes prosobranch snails as the first and a big variety of freshwater fish as the second intermediate hosts. Opisthorchis felineus, O. viverrini and Clonorchis sinensis are species of major medical importance, while sporadic cases of human infection with other representatives of this family are also documented in the literature. There are estimations that 17 million people are infected and more than 350 million people are at risk, mainly in Asian countries. Although opisthorchiid flukes live in the bile ducts of the liver and in the gall bladder, they can affect surrounding liver tissue and even other organs. They are also known to induce the production of autoantibodies as well as allergic reactions. However, the main medical significance is their role in the formation of malignant tumors. Since the clinical symptoms are not specific, a diagnosis must be confirmed by parasitological examination or the detection of antibodies. Apart from coproscopical methods, there are more recent and highly specific assays available, like the detection of coproantigen or the detection of DNA. Praziquantel is the drug of choice for the treatment of opisthorchiidosis. In order to prevent infection, consumption of raw flesh of freshwater fish must be avoided.