We start with role-based trust management (RBTM) and address some of the challenges associated with using RBTM in mobile ad hoc networks (MANETs). We then enhance RBTM with reputation systems (RSs), and propose a new hybrid trust management system (HTMS). In HTMS, the privilege level of an entity is determined not only by its role in the system, but also by its reputation score, which in turn is based on its behavior. If a privileged node becomes compromised and conducts several malicious or risky transactions, then its privilege level is quickly reduced to limit its access to resources and minimize the damage it can inflict further. The system uses a global, network-wide perspective to thwart global attacks. Such fine-grained variations of access control and dynamically assigning privilege levels would be very difficult to accomplish manually. We evaluated HTMS by comparing an implementation of it against an ideal response. We show that HTMS performs very close to the ideal if we can accurately estimate the proportion of malicious nodes in the network. We suggest using sampling to estimate this proportion. However, even if this estimate is not accurate, the results are still much better than using RBTM by itself. EDICS: SYS-ARCH; SYS-PROT; FOR-DETE; SYS-INTR.