When biosensors are operated continuously, a dynamic delay and a dynamic error relate the sensor signal to the changing analyte concentration. The dynamic delay is the temporal displacement of the signal, or the lag, and is specified solely by properties of the biosensor and external mass transfer. The dynamic error is the difference between the actual concentration and the simultaneous reported concentration and is the product of the dynamic delay and the instantaneous rate of concentration change. In real-time operation of sensors, a maximal dynamic error based on the maximal expected rate of concentration change must be employed to estimate the worst-case error because the actual instantaneous rate is not independently known. Values of dynamic delay and maximal dynamic error that are acceptable in particular monitoring situations can be used in the design of acceptable continuous biosensors. This analysis suggests experimental alternatives to the standard response time approach for sensor characterization that are particularly advantageous for continuously operated biosensors. The concepts are applied here to in vitro operation of a continuous glucose sensor.