Overproduction of four primary metabolites by filamentous fungi is explained from a scientific point of view. Causal connections concerning two organic acids, citrate and D-gluconate, the vitamin-like compound, arachidonic acid, and the vitamin riboflavin are explained. While extracellular D-gluconate production by Aspergillus niger is already at 99% yield, recent cloning of a putative citric acid exporter gene might improve productivity in the future. Highest potential for improvement might show arachidonic acid. Its current producer, the zygomycete Mortierella alpina, is underdeveloped concerning molecular tools. Additionally, biochemical reactions and their regulation, e.g., the question how fatty acid chain length and degree of desaturation are controlled, are not understood. In riboflavin overproduction by the hemiascomycete Ashbya gossypii, metabolic engineering begins with an improved uptake of fatty acids into the hyphae. Connected induction of sporulation and riboflavin overproduction is shown and related to the second messenger cAMP. But, the molecular mechanism of induction is not investigated yet.