Syncope is a sudden loss of consciousness associated with loss of postural tone (collapse) from which recovery is spontaneous. The evaluation of syncope in animals is often challenging. Difficulties arise from the very nature of syncope: episodes are usually unpredictably sporadic, sometimes infrequent, and intersyncopal periods are often unremarkable. The common denominator leading to all forms of syncope is decreased or brief cessation of cerebral blood flow. Pre-syncope is a term used to describe episodic hindlimb or generalized weakness, ataxia, or altered (but not complete loss of) consciousness. Compared with syncope, pre-syncope is associated with a less severe or more transient insult that leads to less severe cerebral hypoxia. Severe heart rhythm disturbances are probably the most common cause of syncope in dogs and cats. The chapter considers: Is it syncope?; Causes of syncope; Diagnostic approach; Treatment; and Syncope in cats.