This paper is a comprehensive and up-to-date survey on clutter spatial intensity estimation methods proposed over a span of two decades for radar target tracking. Here, the methods are grouped into three categories. The first category of methods is applicable to measurements in a track validation gate, while the second category of methods compute the clutter spatial intensity of any measurement in measurement space. Finally, the third category uses the concept of clutter generators, which act as the source of clutter measurements. Then, probability hypothesis density filter equations are used to derive the clutter spatial intensity of measurements. The above classification of different methods is based on the techniques used and on the assumptions made while computing the spatial intensity of clutter. This paper emphasizes the underlying ideas and assumptions of each of the methods so that the reader could understand not only how each method works but also their pros and cons. Also, an effort is made to bring out the interrelationship between different methods, wherever possible.