The Horsley-Clarke apparatus is a device invented in 1908 by British neurosurgeon and scientist Sir Victor A.H. Horsley and his colleague Robert H. Clarke at University College London to allow experimental and surgical intervention in deep-seated structures of the brain in vertebrates.

Also called a stereotactic device, the system uses a set of three cordinates (x, y, and z) in an orthogonal frame of reference (cartesian coordinates). In a brain atlas , composed by serial transverse sections of the animal or human brain, each brain structure can be assigned a number of coordinates. In most atlas, the three dimensions are: latero-lateral (x), dorso-ventral (y) and rostro-caudal (z). The mechanical device has head-holding clamps and bars which puts the head of the animal in a fixed position in reference to the coordinate system (the so-called zero or origin). Guide bars in the x, y, and z directions, fitted with high precision Vernier scales allow the experimenter to position the point of a probe (an electrode, a cannula, etc.) inside the brain, at the calculated coordinates for the desired structure, through a small trephined hole in the skull. Then, stimulation, lesions, tissue biopsies, infusion or diffusion of chemical substances, etc. can be done to that spot or area, using a minimally invasive approach.

The original apparatus was developed for the work with small to medium experimental animals. It was only in 1947 that the first stereotactic devices for human neurosurgery were developed, by the American neurosurgeons Ernest A. Spiegel and Henry T. Wycis , and Swedish neurosurgeon Lars Leksell. Thus, stereotactic surgery of the brain was born. They were initially used for surgical treatment of Parkinson's disease (pallidotomy , or the surgical ablation of small areas of the pallidum , an area in the basal ganglia of the brain, which are implicated in the physiological control of movement), but later found many applications in the surgical treatment of tumors, vascular malformations , aneurysms, and abscesses, as well as in functional neurosurgery , such as epilepsy, chronic pain, and mental health (psychosurgery).