Basal ganglia (BG) pathology is observed in many disorders. This chapter focuses on an analysis of clinical symptoms. Malfunctioning in the control loop can result in different symptoms depending on the component damaged. Input function damage can disrupt feedback and lead to oscillations and uncontrollable outputs, while excessive gain can result in instability in control. Reduced transition gain causes movement velocity and tempo reduction, as seen in akinesia and bradykinesia. Asymmetric position reference signals are responsible for deviant postures. Impaired feedback in repetition control contributes to perseveration and stereotypy in disorders such as autism, obsessive–compulsive disorder, and Tourette’s syndrome. In psychiatric disorders, hallucinations can arise due to excessive BG activation of perceptual representations in visual and auditory cortices. Schizophrenia patients also exhibit impaired action–outcome learning and monitoring due to malfunctioning of the associative cortico-BG network, including the dorsolateral prefrontal cortex and associative striatum. Disrupted efference copy signaling in this network may contribute to delusions by impairing agency detection.