After a brief discussion on the chemical-nutritional characteristics of the diets currently used in aquaculture, the authors focus attention on the problems associated with the replacement of fish meals with vegetable protein sources. The chemical composition of vegetable sources has a high variability and it is necessary to know the exact energy content and digestibility of amino-acids for each source. Another question is the occurrence of anti-nutritional factors in vegetable protein sources and their effect on diet palatability. The authors also underline that the use of high levels of vegetable sources increases the risk of contamination by mycotoxins of which the effects on the health of fish are not yet completely clear. Finally, brief considerations are reported on the relationship between the chemical-nutritional characteristics of diets and that of the resulting fish fillets.