A yeast cell introduced into an aqueous solution of glucose and ammonium salts will use the nutrients for biosynthesis and growth under suitable environmental conditions such as pH and temperature. If, alongside ammonium salts, amino acids were supplied, the yeast cells would stop absorbing the ammonium salt and instead utilize the supplied ‘readymade’ substrate. If a starch-utilizing yeast was supplied nothing but starch and ammonium salts, it would secrete extracellular enzyme(s) to breakdown the starch to sugars. These sugars would then be absorbed and would be used with ammonium salts for biosynthetic activities. The organism is not only able to ‘decide’ when to manufacture and secrete certain enzymes to enable it to utilize materials in the environment, but it is able to decide to stop the synthesis of certain compounds if they are supplied to it. Therefore, knowledge of these regulatory mechanisms and biosynthetic pathways is essential to enable the industrial microbiologist to derange and disorganize them so that the organism will overproduce desired materials. This chapter presents regulatory mechanisms for primary and secondary metabolic products through enzyme regulation, substrate induction, catabolite regulation, feedback regulation, permeability control, derangement or bypassing of regulatory mechanisms.