To provide for reliability in Wireless Sensor Net- works (WSNs), Medium Access Control (MAC) protocols must be adapted by mechanisms taking cross-layer approaches into account. We describe AreaCast which is designed for enhancing reliability in WSNs. AreaCast is a MAC layer mechanism inde- pendent of the routing layer, but uses only local topological and routing information to provide a communication by area instead of a traditional, node-to-node communication (i.e., unicast). In AreaCast, a source node addresses a set of nodes: an explicit relay node chosen as the next hop by a given routing protocol, and k other implicit relay nodes. The neighboring nodes select themselves as implicit relays according to their location from the explicit relay node. This mechanism uses overhearing to take advantage of the inherent broadcast nature of wireless commu- nications. Without changing the routing protocol, AreaCast is able to dynamically avoid a byzantine node or an unstable link, allowing to benefit from the inherent topological redundancy of densely deployed sensor networks. Simulation results show that AreaCast significantly improves the packet delivery rate while having a good reliability-energy consumption trade-off.