The continuous fermentation offers in comparison to the discontinuous process a number of advantages. These include the long-term operation with a largely constant substrate, cell mass and product concentrations and high plant productivity. In contrast there is a more sophisticated technique and the need to secure long-term sterility. First of all the basics of the continuous fermentation for starting operation and steady-state operation are treated. A consistently applied dimensionless formulation of the relevant relationships enables a compact and very clear presentation of the basic laws for the cell mass and product formation. The special features of the continuous fermentation with and without cell retention and fundamental aspects of the continuous fermentation with cell retention are discussed.