Abstract: Many modern games and game systems allow for networked remote participation. In such networks latency variability is a commonly encountered factor, but there is still little information available to designers about how human performance changes in the presence of delay. To add to our understanding of performance thresholds for mouse-based tasks that are common in real-time games, we carried out a study of human target acquisition and target tracking in the presence of latency and jitter (variance in latency), for various target velocities and trajectories. Our study indicates critical thresholds at which human performance decreases in the presence of delay. Target acquisition accuracy drops very quickly for latencies over 50 ms and for high velocities. Tracking error, however, is only slightly affected by latency, with deterioration starting at around 110 ms. The effects of latency and target velocity on errors are close to linear, and transverse error is usually smaller than longitudinal error. These results help to quantify the effects of delay on closely-coupled interactive tasks in networked games and real-time groupware systems. They also aid designers in determining when it is critical to improve system parameters and when to apply prediction and delay-compensation algorithms to improve quality of interaction.