Copper and its alloys generally display a severe reduction in ductility between roughly 300 and 600 degrees C, a phenomenon variously called 'intermediate temperature embrittlement' or 'ductility trough behaviour'. This review of the phenomenon begins by placing it in the wider context of the high-temperature fracture of metals, showing how its occurrence can be rationalised in simple terms on the basis of what is known of intergranular creep fracture and dynamic recrystallisation. Data in the literature are reviewed to identify main causes and mechanisms for embrittlement, first for pure copper, and then for monophase and multiphase copper alloys. Coverage then turns to the 'grain boundary embrittlement' phenomenon, caused by the intergranular segregation of even minute quantities of alloying additions or impurities, which appears to worsen dramatically the intermediate temperature embrittlement of copper alloys. Finally, metal-induced embrittlement, including in particular liquid metal embrittlement, is presented as a second mechanism leading to an exacerbation of the intermediate temperature embrittlement of copper and its alloys.