Increasingly, commercial content providers (CPs) offer streaming and IPTV solutions that leverage an underlying peerto-peer (P2P) stream distribution architecture. The use of P2P protocols promises significant scalability and cost savings by leveraging the local resources of clients – specifically, uplink capacity. A major limitation of P2P live streaming is that playout rates are constrained by the uplink capacities of clients, which are typically much lower than downlink capacities, thus limiting the quality of the delivered stream. Thus, to leverage P2P architectures without sacrificing the quality of the delivered stream, CPs must commit additional resources to complement those available through clients. In this paper, we propose a cloud-based service – AngelCast – that enables CPs to elastically complement P2P streaming “as needed”. By subscribing to AngelCast, a CP is able to deploy extra resources (“angels”), on-demand from the cloud, to maintain a desirable s...