Economics has used Neoclassical-Pareto Efficiency Theory as economic efficiency for a long time. However, this theory is not sufficient to explain the (in) efficiencies of the real economic activity. Although some approaches have introduced against the conventional theory, a more general and comprehensive explanation on efficiency is that engaged by modern economic theory has not yet been in force. The dimensions of ongoing controversy has reached from X (in) efficiency approach to the modern theories of the firm. Departing X (in) efficiency approach, the aim of this essay is to study the sources of efficiency existing in the real economic life and to model its determinants within today’s market relations.