Given that wireless communication occurs in a shared and inherently broadcast medium, the transmissions are vulnerable to undesired eavesdropping. This occurs even when a point-to-point communication is sought, and hence a fundamental question is whether we can utilize the wireless channel properties to establish secrecy. In this paper we consider secret communication between two special nodes ("source" and "destination") in a wireless network with authenticated relays: the message communicated to the destination is to be kept information-theoretically (unconditionally) secret from any eavesdropper within a class. Since the transmissions are broadcast and interfere with each other, complex signal interactions occur. We develop cooperative schemes which utilize these interactions in wireless communication over networks with arbitrary topology, and give provable unconditional secrecy guarantees.