Security is a paramount concern in Mobile Ad hoc NETwork (MANET) because of its intrinsic vulnerabilities. These vulnerabilities are nature of MANET structure that cannot be removed. As a result, attacks with malicious intent have been and will be devised to exploit these vulnerabilities and to cripple MANET operations. In this paper, the authors analyze the security problems in MANET and present a few promising research directions. On the prevention side, various key and trust management schemes have been developed to prevent external attacks from outsiders, and various secure MANET routing protocols have been proposed to prevent internal attacks originated from within the MANET system. On the intrusion detection side, a new intrusion detection framework has been studied especially for MANET.