Abstract: Around the three basic problems of firm theory,almost all kinds of firm thcories, including labor division theory,neoclassical firm theory,entrepreneur theory,maager theory, contract theory,stakeholder theory,ability theory and knowledge theory,are expounded in the economic and managerial point of view.Then,the existence and develoof small and medium-sized enterprises are interpreted byimportant these of those firm theories, on the basis of which the theory of small and medium-sizedterprises has been formed.