Spasticity is usually observed along with paralysis, hyperreflexia, Babinski sign and abnormal associated movements associated with dysfunction of central motor tracts. In spasticity, exaggeration of the stretch reflex results in increased resistance during passive movements. Therefore, spasticity is pathophysiologically described as increased muscle tone whose pathognomonic sign is decreased passivity. Resistance is more strongly felt during rapid passive movements than during slow movements. The resistance felt at the beginning of the passive movement abruptly diminishes, which is well known as the clasp-knife phenomenon. Another character of spasticity is the distribution of the increment in the muscle tone. Not only rigidity, dystonia, and muscle stiffness demonstrating increased muscle tone, but also Gegenhalten and contracture of joint with normal muscle tone should be differentiated. No neurophysiological parameters reflect the degree of spasticity in a strictly parallel fashion. However, neurophysiological examinations provide some supportive objective data. Surface electromyography is useful to distinguish spasticity from rigidity and other conditions with increase muscle tone. The increased amplitude ratio and the decreased threshold ratio of the H-wave to the M-wave, and increased amplitude and persistence of the F-wave are observed the patients with spasticity. Magnetic stimulation is a useful tool to detect corticospinal tract lesions that induce leading to spasticity. Transcranial magnetic stimulation, magnetic brainstem stimulation, and magnetic spinal motor root stimulation are used to examine the entire motor pathway. Since positive correlation between spasticity and shortening of the silent period is reported, many investigations including paired-pulse magnetic stimulation will be necessary for understanding pathophysiology of spasticity. Patients with mild and reversible spasticity are usually treated with medications. Significant variations exist in the use of these therapies, because the treatments often depend on the clinicians' experience. It will be necessary to clarify the action mechanism of drugs, to develop new effective drugs, and to perform randomized controlled trails so that clinicians can select the optimal medication based on evidence.