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The claws and claw folds are highly specialized structures. The claw grows very slowly (0.8–1.9 mm per week) from a particular structure inside the claw fold, called the matrix, which is in direct contiguity with the epidermis of the adjacent skin. The matrix surrounds the claw; however, the dorsal matrix is more active than the ventral one and this determines the curved aspect of the claw in dogs and cats. Any alteration of the matrix results in deformation of the claws. Anatomically, the claw is composed of three portions: one dorsal surface, two lateral surfaces and one interior surface. Clinically, the claw appears smooth, hard in consistency and with variable, but homogenous colour (e.g. white translucent, pink or dark). The length of the claw is determined by its intrinsic growth, the environment, and the species or breed. For example, small-breed dogs that live almost entirely indoors need more frequent trimming of their claws owing to the lack of natural usage. The chapter considers Differential diagnosis; Clinical approach; and Treatment.