The analysis in this book, so far, has concentrated on using sensors to detect the presence of targets in noise and clutter. However, sensors are capable of much more than that. Consecutive measurements of a target's position (and sometimes velocity) can be used to calculate the target state (position, velocity, and acceleration), from which a good estimate of the future position can be made. This process of estimation is generally referred to as tracking. For most time-of-flight sensors that operate in polar space, this involves following the target independently in both range and angles to obtain good estimates of its position in three dimensions. This chapter is concerned with the mechanics of the tracking process.