The principal aim of this chapter is to introduce the reader to clinical applications of color assessment and to describe how knowledge of color vision loss is used in clinical practice. The chapter starts with a review of important aspects of normal, trichromatic color vision (Sect. 11.1). Then, pathophysiological mechanisms that underlie color vision defects are discussed (Sect. 11.2), followed by an introduction to the most important principles of color vision testing in clinical practices (Sect. 11.3). Color vision defects can be classified in a clinically meaningful way based on these results (Sect. 11.4). A detailed classification of the retinal, optic nerve, and systemic diseases that can cause loss of color vision follows (Sect. 11.5). Emphasis is placed on the detection of small changes in color vision that can precede the earliest clinical signs of a disease, the classification of the patient’s class of color vision (i.e., normal trichromacy, deutan, protan, tritan, or acquired deficiency), and the test’s ability to quantify the severity of color vision loss.