Different types of product formation kinetics are discussed with respect to their significance for fermentation process economics. Microbial products belonging to various classes are formed in a growth-coupled manner. It is often found that the specific rate of product formation increases with the specific growth rate, approaching a maximum at higher growth rates. It is illustrated that for such types of relationship between the product formation rate and the growth rate process conditions are optimal when the specific rate of product formation is about half-maximal.