This chapter extends the discussions of optical effects to those due to third-order susceptibilities X(3). The mechanism for the X(3) nonlinearity is attributed to distortions in the electron cloud surrounding an atom or molecule. The chapter investigates X(3) processes that are electronic in origin. The corresponding nonlinear phase shifts lead to phenomena such as self-focusing, modulation instability, and solitons. The chapter then introduces the nonlinear polarization for X(3) processes, which are proportional to the cube of the input field. It also shows how a X(3) material can be used in a resonator geometry to make devices that can have two 240 (or more) stable output intensities for the same input intensity; a phenomenon called optical bistability. Finally, the chapter discusses degenerate four-wave mixing (DFWM) and a technique called z-scan, which is a method to measure the real and imaginary coefficients of the X(3)nonlinearity of a medium.