Abstract Three-dimensional object recognition is complicated by the fact that an object may present itself to a sensor in any one of an infinity of poses. Fortunately, this problem can be alleviated by declaring equivalent classes of object poses through either the use of the aspect graph representation of objects [7, 14] or through the use of local feature sets (LFSs) [3, 9]. To elaborate a bit on the notion of local feature sets, since they are so central to the work described in this chapter, consider the two simple model objects shown in Figure 2.l(a) and 2.l(b). Each surface of each of these objects is called a feature and each feature can be characterized by a set of attribute-value pairs. For example, the surface features 9 and 4 of an instance of the object in Figure 2.l(a) can be described by the attribute-value pairs: How attributes are used in matching a feature from the scene with a feature from a model depends on the nature of the attribute. If an attribute, such as “area,” is susceptible to occlusion, then it can be used only in the sense of its providing an upper-bound constraint, meaning that we must exclude all model features whose areas exceed the area of the feature in the scene. This point is discussed in greater detail in [8].