This chapter aims to provide a comprehensive picture of the impact of fiber nonlinear effects on modern coherent wavelength division multiplexing (WDM) systems’ performance. First, the main nonlinearity models currently available are introduced and discussed in depth. Then, various specific aspects are addressed, such as the interplay of polarization mode dispersion (PMD)/polarization dependent loss (PDL) and nonlinearity, or the dependence of nonlinear effects on modulation format. The important topic of nonlinear effects mitigation is then dealt with. Finally, system performance metrics and capacity are discussed extensively, as to how they are fundamentally influenced and limited by fiber nonlinearity.