This paper describesi a simple rate-distortion model, shows how the model/ parameters can be estimated us-ing Kalman filtering,/ and suggests how this information may be used for $-ate control in real-time systems. Real-time parameter estimation is more robust with the simplified model, and the model yields useful insights. Also, the suggested kalman filter is computationally efficient, and adapts to changes in the video sequence. Several uses of the model and parameter estimates are suggested for real-time rate control.