Wireless sensor and ad hoc networks are gaining a lot of attention in research lately due to their importance in enabling mobile wireless nodes to communicate without any predetermined infrastructure. Routing protocol in wireless sensor and ad hoc networks discover a multi-hop route between source and destination nodes. This paper presents RAS: a Reliable routing protocol for wireless Ad hoc and Sensor networks. In the RAS protocol, increased reliability isachieved by the maintenance of a reliability factor by the nodes. The value of this factor is increased when nodes participate successfully in data transmissions. This is determined through the use of positive and passive acknowledgements. During the path discovery process, an intermediate node only extends the request message to nodes that have a minimal reliability factor which is specified by the source. Additional optimizations are included in order to increase the efficiency and performance of the network.