Two apparently separate areas of medical science, head and neck and neuroscience, are often combined in the early phases of undergraduate medical education. Perhaps an obvious reason for this is that the brain, together with the organs of special sense — eyes, ears, nose, and taste buds — are located in the head. Head and neck injuries can therefore be serious and are commonly life-threatening. Another reason is embryological. The development of the head and the central nervous system (CNS) are closely intertwined. The whole CNS is essentially a segmented structure, with a pair of spinal (or cranial) nerves arising in each body segment. For the spinal cord and spinal nerves, each segment is marked by its own vertebra. The situation is more complex in the head, where the developing brain undergoes cervical, cephalic, and pontine flexures. These folds in the growing neural tube, plus the development of a protective cranium, obscure the underlying segmental pattern, but each segment of the brain still bears its pair of cranial nerves. The organization of the CNS and peripheral nervous system is complex but ordered, and neurological disorder can often be diagnosed by a process of clinical reasoning if the structural and functional properties of the system are sufficiently well understood. Neurological disorders commonly present as alteration in, or loss of, sensation or disturbance of motor function. Knowing which areas of skin (the dermatomes) and which muscles are innervated by each cranial or spinal nerve, together with understanding the characteristic deficiencies produced by abnormality, will often allow the neurologist to use clinical reasoning skills to localize a lesion with considerable accuracy, before radiological or other investigation is undertaken. The diagnostic process is assisted by specific neurological tests, performed during the physical examination, which investigate the integrity of various neural pathways. Disorders of the CNS can involve alterations of sensory perception, motor performance, emotion, overt behaviour, consciousness, and perceptions of self. Some diagnoses may be made with neurological techniques, others by psychiatric techniques, and in many instances the recognition of characteristic patterns of altered perception, performance, or behaviour may be important clues.