This chapter presents an overview of the set of symptoms generally classified as spasticity. Definitions of spasticity are reviewed, and this chapter suggests the use of a recently presented definition, “disordered sensori-motor control, resulting from an upper motor neurone lesion, presenting as intermittent or sustained involuntary activation of muscles.” Following a brief discussion of mechanisms, assessment of spasticity is organized by consideration of the domains of the international classification of functioning, disability, and health (ICF) and clinical and laboratory scales reviewed.