In terms of its size and number of neurons, the subthalamic nucleus (STN) is a relatively minor component of basal ganglia, but its strategic position and the fact that it is the only primarily glutamatergic nucleus of the basal ganglia and their 'driving force', makes it significant. The STN is the main nucleus of the subthalamus, a part of the diencephalon, which it also develops from. It is considered a tripartite nucleus composed of a dorsolateral sensorimotoric region, a ventromedial associative region and a medial limbic region, but studies that featured electrical lesions, anterograde and retrograde tracers, classical cytoarchitectonics and magnetic resonance imaging (MRI) produced results that supported a topological organization without strict anatomical boundaries within the nucleus. The main afferents to the STN arise from the cortex and the external segment of the globus pallidus (GPe), and the main efferents are projected reciprocally to the GPe and to the output nuclei of the basal ganglia, the internal segment of the globus pallidus (GPi) and the substantia nigra pars reticulata (SNr), through which the basal ganglia control voluntary movement. Basal ganglia are also the site of pathology in Parkinson's disease (PD) which is caused by the relative dominance of the indirect pathway which relays cortical and thalamic excitation indirectly to the output nuclei, involving the GPe and the STN, and suppresses movement. In PD, activity within and across the nuclei is low frequency and coherent, so high frequency deep brain stimulation (DBS) is used as adjuvant therapy. Aside from relieving motor symptoms, DBS causes side-effects in the form of behavioural, psychiatric and cognitive changes, which suggests that the STN has various functions. Research has shown that the STN elevates the decision threshold, is involved in the process of motor learning, has opposite roles in terms of motivation for natural rewards and drugs of abuse and is involved in emotional processing and regulation of behaviour, which emphasizes its significance in the function of the basal ganglia.