Mobile ad hoc networks (MANETs), due to their unique characteristics (e.g., unsecured wireless channel, dynamic mobility, absence of central supportive infrastructure and limited resources), are suffering from a wide range of security threats and attacks. Particularly, MANETs are susceptible to the Denial of Service (DoS) attack that aims to disrupt the network by consuming its resources. In MANETs, a special form of DoS attack has emerged recently as a potentially major threat: the flooding attack. This attack recruits multiple attack nodes to flood the MANET with overwhelming broadcast traffic. This flooding traffic is so large that all, or most of, MANET resources are exhausted. As a result, the MANET is not able to provide any services. This thesis aims to investigate the flooding attack and propose detective security measures to defend MANETs against such an attack.