This chapter provides a simulation model for four typical mobile ad hoc network (MANET) routing protocols to study the performance of Ad Hoc On-Demand Distance Vector (AODV), Dynamic Source Routing (DSR), Temporally Ordered Routing Algorithm (TORA), and Optimized Link State Routing (OLSR) and validates simulation results. It compares the performance of commonly used MANET routing protocols. the chapter discusses the effect of node mobility on the performance of MANET routing protocols. It describes the effect of traffic loads on the performance of MANET routing protocols and analyses the effect of network size on the performance of MANET routing protocols. The end-to-end packet delay, throughput, routing load, and retransmission were measured to evaluate the performance of OLSR, AODV, DSR, and TORA. An OPNET-based model is developed to study the impact of routing protocols on a typical 802.11b MANET performance. The number of nodes in a network plays an important role when it comes to the performance of routing protocols.