Spasticity is a neuromuscular dysfunction characterized by tight or stiff muscles. Spasticity occurs across the spectrum of upper motor neuron disease and complicates the course and quality of life of those affected. Accurate and precise assessment of spasticity is the first step in providing safe and effective treatments to patients for management of spasticity. Examiner evaluations (Ashworth Scale, Modified Ashworth, and Visual Analog Scale) and patient self-reports (Visual Analog Scale and Numeric Rating Scale) are used to assess spasticity in clinical practice. We reviewed the biology of spasticity and summarized research that assessed properties of scores obtained from clinical scales when used in a variety of upper motor neuron diseases. The definition of spasticity was inconsistent. Rater reliability or agreement on clinical scales varied widely. Correspondence with electromyogram results was mixed. There was dissimilarity in patient reports and examiner assessments. Scores from clinical scales are responsive (decrease after initiation of treatment with known effectiveness), but the utility of scores for indexing individual change associated with the natural history of upper motor neuron disease is unknown. Future research incorporating patient reports and examiner findings over time will help to clarify the definition and capture the essence of spasticity.