This work analyzes cooperative networks in which a single source message is retransmitted by multiple levels of amplify-and-forward (AF) relays that are equipped with single antennas. The effects of noise and signal accumulation and multihop diversity (combining signals from multiple previously transmitted levels) on the network performance are studied. This work quantifies the advantages of multi-level networks over the classical multihop networks, which has degrading performance as the size of the network increases when relays use the simple AF strategy.