In a typical wireless network, users exploit their available power and bandwidth resources to communicate their messages to their target destinations as efficiently as possible, and ignore or avoid the messages of other transmitting users. However, we show this selfish strategy is, in fact, not in the collective interest of the group: through cooperation each individual user can experience better performance. In particular, if each user devotes some of its resources to relaying the transmissions it hears from other users as well as to sending its own message, each message is received with higher reliability because it experiences multiple propagation paths. Indeed, with this protocol, the group of cooperating users are forming a "virtual" antenna array, from which there is a substantial spatial diversity benefit. We show that, in fact, very simple relaying schemes yield full spatial diversity: each user's message experiences performance as if it were sent from a physical antenna array of the same size. These lead naturally to novel and efficient distributed space-time codes for networks, which are equally applicable to cellular and ad-hoc architectures. From an architectural perspective, such protocols require only a relatively small violation of the traditional abstraction rules that impose a separation of the physical, link, and network layers.