Fundamentals of the nonlinear theory of Fabry-Perot semiconductor lasers have been developed, an integral part of which is the natural linewidth theory. The formula for gain depending on the energy flux specifies the basic nonlinear effect in a laser. Necessary conditions for stimulated emission of the first and second kind are presented. Maxwell's equations in the gain medium are used to obtain equations for energy flux and for non-linear phase effect description. Based on the nonlinear theory, a number of experiments have been simulated to suggest that the developed nonlinear theory is a new paradigm of the laser theory. The nonlinear theory has provided recommendations for the development of lasers with improved properties, such as lasers with increased power and lasers with reduced natural linewidth.