Spasticity, a frequently encountered symptom in patients with upper motor neuron (UMN) syndrome, poses a significant challenge, negatively affecting function, activity, and social engagement. Despite the acknowledged benefits of exercise in the rehabilitation of UMN syndrome, therapy sessions often trigger an unwelcome increase in muscle stretch reflex activity, resulting in considerable muscle tension despite improvements in function and activity levels. Despite the recognized benefits of exercise in UMN syndrome rehabilitation, there's often an undesirable rise in muscle stretch reflex activity during therapy, leading to considerable muscle tension despite improvements in function and activity levels. The challenge lies in identifying effective strategies that enhance function, activity, and participation while curbing excessive muscle tension caused by heightened stretch reflex activity. Spasticity significantly disrupts the daily lives of affected individuals and presents substantial challenges for caregivers. However, existing methods for measuring and evaluating spasticity have their limitations and are susceptible to errors. This article describes both established and innovative methods utilized for quantitative spasticity assessment and management of spasticity, with the overarching goal of improving the definition of spasticity and identifying assessment techniques suitable for clinical application.