Edible fish and crustaceans, either of salt or of freshwater origin, are occasionally affected by off-flavours that can be related to the animal's diet or the natural environment in which they live. Off-flavours include petroleum and blackberry-like flavours in salmon and cod; garlic-like flavours in prawns and sand-lobsters; iodoform-like flavours in prawns, shrimp and fish; and muddy and earthy flavours in brackish-water fish and shrimp. Knowing the source of such off-flavours will in some cases assist processors in taking remedial action. This paper will discuss the possible biochemical pathways to five of these off-flavour compounds and, in addition, will provide evidence as to the likely marine organisms responsible for their formation. Methods to reduce the severity of such off-flavour incidents will also be described.