Best-effort delivery describes a network service in which the network does not provide any guarantee that data is delivered or that delivery meets any quality of service. In a best-effort network, all users obtain best-effort service. Under best-effort, network performance characteristics such as network delay and packet loss depend on the current network traffic load, and the network hardware capacity. When network load increases, this can lead to packet loss, retransmission, packet delay variation, and further network delay, or even timeout and session disconnect.

Best-effort can be contrasted with reliable delivery, which can be built on top of best-effort delivery (possibly without latency and throughput guarantees), or with virtual circuit schemes which can maintain a defined quality of service.