Recently a great deal of attention has been paid to Mobile Ad Hoc Networks (MANETs) due to their important roles in many different applications that include, but are not limited to: civil, military, and health applications. This kind of network can reach places that difficult for human beings to reach such as disaster areas. In spite of the rapid development of MANET technologies, the routing process still poses a real challenge due to the high mobility and dynamic topology features of such networks. This paper presents an investigation into some popular MANET routing protocols. This investigation aims to evaluate the performance of MANET routing protocols using the key performance indicators: throughput, end to end delay, and data packet delivery ratio. These indicators are commonly used in such evaluations. Our investigation results showed that DSR and AODV outperform DSDV. In a high-density network (a network with a large number of nodes), AODV outperforms DSR; while, in a low-density network the DSR performs better than the AODV. So, DSR is applicable for small networks; while AODV is applicable for large networks.