This paper defines a new group communication model called concast communication. Being the counterpart to multicast, concast involves multiple senders transmitting to a single receiver. Concast communication is used in a wide range of applications including collaborative applications, report-in style applications, or just end-to-end acknowledgements in a reliable multicast protocol. This paper explores the issues involved in designing concast communication services. We examine various message combination methods including concatenation, compression, and reduction to reduce the traffic loads imposed on the network and packet implosion at the receiver. Group management operations such as group creation/deletion, joining/leaving, and concast routing are discussed. We also address transmission issues such as reliable delivery, flow control, congestion control, and QoS. We conclude the paper by presenting a concast communication model that we have been developing in the context of TMTP5. The model uses concast communication to implement reliable multicast and it shares concast trees with the multicast group whenever possible to reduce overhead costs.