Borderline between the primary and secondary metabolism is not very sharp. Secondary metabolism is discussed as a specific metabolic process that does not necessarily occurs during the whole life of organism. Its specificity is given by its probably minor side position in the economy of organism. The secondary metabolism involves synthetic processes producing secondary metabolites of various structures without obvious function in the producing organism. Some secondary fungal metabolites are beneficial (antibiotics such as penicillin) while others are harmful (carcinogens such as sterigmatocystin and aflatoxins).