The impact of relay nodes on the secret key generation via the physical layer resources is investigated. A novel relay-assisted strategy is proposed to improve the generated secret key rate. The main idea is to exploit the random channels associated with relay nodes in the network as additional random sources for the key generation. This approach is particularly useful when the channels between legitimate nodes change slowly. Three increasingly sophisticated yet more practical scenarios are studied, for which relay-assisted key generation protocols are proposed and are shown to be optimal or order-optimal in terms of the key rate. It is also shown that the presence of relay node increases the multiplexing gain. This is in sharp contrast to scenarios with relay helping information transmission, in which the relay node does not increase the multiplexing. Furthermore, a cooperative scheme is also proposed in which relays help key generation but the generated keys are kept secure from these relays.