In this chapter, the authors explain nonlinear phenomena that occur in ferromagnetic samples in the presence of a single, in particular, the uniform mode. Equilibrium (thermal) spin waves with small amplitudes and with frequencies distributed over broad range always exist in magnetically ordered substances. Due to the nonlinearity of the spin system they are coupled to the uniform magnetization mode excited by the external ac field. When the amplitude of this (pumping) mode exceeds certain threshold value, the energy acquired by spin waves from this mode compensates spin-wave losses, and the instability, i.e., the exponential growth of the spin-wave amplitudes, begins. Nonuniformities (defects), as impurities, pores, surface roughness, and grain boundaries in polycrystals, lead to the so-called two-magnon processes, i.e., to the scattering of magnetic oscillations and waves by defects. The main assumption is the neglect of interaction between spin waves in different domains.