A general modeling and simulation strategy suitable for the fast and accurate analysis of a fiber-optical WDM system is presented, that may also include multi-span systems. Noise and fiber dispersion are considered as well as nonlinear effects like four wave mixing, self-phase modulation and cross-phase modulation. Furthermore, amplified spontaneous emission noise of the optical amplifiers and polarization-dependencies (e.g. PMD) are taken into account. Performance evaluation by means of eye patterns, spectral power densities, optical signal-to-noise ratio, the Q-Factor and the bit error rate are addressed. An analysis of the degradation effects against the position within the fiber is shown to get a better overview of the fibers behavior. An improved Split-Step algorithm is outlined as a fast alternative to supplement the FFT calculation within the fiber. A parameter variation, which can also be influenced during the simulation by the user, is presented in order to get an overview of the parameter space. Different modulation formats are taken into account, e.g. return-to-zero, non-return-to-zero and differential phase-shift keying. Both the separated channels and the total field approach are demonstrated. In the separated channels approach the different nonlinear effects can be switched on and off independently for detailed studies of inter-channel effects. Based on this work, a complete design environment (PHOTOSS, The Photonic System Simulator) has been developed. This simulation tool has been tested extensively by several industry partners. Simulation examples are presented here e.g. for PMD simulations.