Mobile ad-hoc networks (MANETs) comprise computer nodes which communicate over wireless links without any central control. Therefore, they must be able to make fully autonomous security decisions. This introduces new security challenges that existing security models and mechanisms do not adequately address (Campbell, 2002). In this paper we present a trust-based security system that deals with the specific challenges of MANETs by combining decentralised security management and context-aware computing. With this combination, our trust-based security system can establish appropriate trust levels for every situation.