Peculiarities of nonlinear interaction between high-power femtosecond laser pulses and molecular atmosphere are analyzed. The peculiarities are connected with combined action of elementary nonlinear effects such as multiphoton or tunnel ionization of molecules, Kerr nonlinearity, and nonlinear resonance absorption by gas components of the atmosphere. Particular emphasis is placed on the possible mechanisms of nonlinear effects of the lowest orders affecting the femtosecond laser pulse propagation along atmospheric paths.