In this chapter, we examine several concepts related to local feature descriptor design—namely local patterns, shapes, spectra, distance functions, classification, matching, and object recognition. The main focus is local feature metrics, as shown in Fig. 4.1. This discussion follows the general vision taxonomy that is presented in Chap. 5 and includes key fundamentals for understanding interest point detectors and feature descriptors, as surveyed in Chap. 6 , including selected concepts common to both detector and descriptor methods. Note that the opportunity always exists to modify as well as mix and match detectors and descriptors to achieve the best results.