Unlike traditional mobile wireless networks, ad hoc networks do not rely on any fixed infrastructure. Nodes rely on each other to route packets to other mobile nodes or toward stationary nodes that may act as a gateway to a fixed network. Mobile nodes are generally assumed to participate as routers in the mobile wireless network. However, blindly trusting all other nodes to respect the routing protocol exposes the local node to a wide variety of vulnerabilities. Traditional security mechanisms rely on either the authenticated identity of the requesting principal or some form of credentials that authorise the client to perform certain actions. Generally, these mechanisms require some underlying infrastructure, e.g., a public key infrastructure (PKI). However, we cannot assume such infrastructures to be in place in an ad hoc network. In this paper we propose an extension to an existing ad hoc routing protocols, which selects the route based on a local evaluation of the trustworthiness of all known intermediary nodes (routers) on the route to the destination. We have implemented this mechanism in an existing ad hoc routing protocol, and we show how trust can be built from previous experience and how trust can be used to avoid routing packets through unreliable nodes.