The processes that lead to formation of large secondary disturbances are different for sheared channel flows and falling films largely as the result of different linear celerity and growth behavior. Close to neutral stability, sheared films usually have a band of linearly unstable wave modes with wavenumbers bounded away from O with a maximum growth rate at frequency omega(m). These conditions have been observed to produce steady moderate wavelength waves with frequency close to omega(m) that remain at small amplitude. For more severe conditions,long waves can become unstable although their growth rate is significantly lower than that at omega(m) and there still may be an intermediate band of stable waves. However, under some conditions, the linear dispersion relation stipulates that a low frequency unstable mode, omega(l), is resonant or nearly resonant with the fastest growing mode, omega(m)(omega(l) much less than omega(m)). Experiments suggest that this mechanism may trigger significant growth of the omega(l) mode by feeding additional energy from the faster growing mode. This mechanism selects the frequency of the low mode and enhances its growth rate. This causes the large disturbances that eventually form to be of a particular frequency. For unstable inclined films, all the wavenumber modes less than the peak are always unstable and the possibility of resonance does not exist because the speeds do not match. Thus no specific frequency is selected. Evolution towards solitary waves, which involves a nonlinear phase-locking mechanism of all unstable modes, occurs under all conditions due to the absence of resonant energy transfer from the most unstable modes. Furthermore, there is no specific frequency selected so that there is no preferred separation for the solitary waves that are formed.