This chapter discussed optical network system-level designs. We initially considered power budget-based design, from which we migrate to more complex OSNR-based designs. This chapter showed the importance of OSNR in estimating BER and the need for evaluation of the Q-factor as an intermediate stage in BER development. We discuss dispersion-based systems and penalties that are associated with dispersion-limited systems. We show how dispersion-limited systems can be compensated using generic schemes and hence show the methods of pre- and postcompensation as well as placement of such compensating units. Nonlinear effects are also studied from a design point of view, as well as the various penalties and their cures from a system design perspective. Finally, we analyzed numerical examples for the studied system design principles.