It is important to obtain strong nonlinearities under the conditions of weak-light through electromagnetically induced transparency (EIT), which opens up a new field about optical nonlinearities, named as weak-light nonlinearity. Kerr nonlinearities and four-wave mixing based on electromagnetically induced transparency are introduced. Kerr nonlinearities may be used on the all-optical switches and all-optical logical arithmetic and optical quantum logical gate and so on. Four-wave mixing effects also have many applications. The most important application is that it can expand the frequency of coherent light to infrared and ultraviolet spectrum. In the case of degeneration, four-wave mixing effects are useful for wavefront reconstruction in the adaptive optics. Besides, in material research, the resonant four-wave mixing technique is a vigorous spectrum analyzing tool.