This paper reports on our theoretical and experimental study comparing the merits and demerits of the nonlinear effects in optical fibers for their use in high-speed and high-capacity time-division-multiplexing (TDM) and wavelength-division-multiplexing (WDM) telecommunication applications. Nonlinear effects include self-phase modulation (SPM), cross-phase modulation (CPM), stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), four wave mixing (FWM), parametric generation, and photorefractive effects. Nonlinear effects can be useful in enhancing the fiber performances by way of fiber lasers, amplifiers, switches, logic devices, modulators, multiplexers, demultiplexers, signal format conversion devices, and wavelength conversion devices, but they can also play degenerative roles limiting the performances of optical fiber communication. The trade-off between the two aspects of the nonlinear effects, therefore, should be carefully examined to achieve a system of desired transmission capacity, performance, and function.