The present invention provides an adjustable cheek rest for installation on the butt stock of a firearm, the cheek rest having an elongate comb adapted to conform to the top portion of the butt stock, the comb being supported on a rail that is fixed to, or removably mounted to, one side of the butt stock. The cheek rest comb is adjustable both longitudinally and vertically with respect to the butt stock to suit the needs of a particular shooter, the comb being attached to the butt stock by a fixture slidably engaging the rail to permit longitudinal adjustment and slidably engaging the comb to permit vertical adjustment. Once adjusted, the comb may be secured in a desired longitudinal and vertical location. A removable rail is also provided for mounting the cheek rest and/or other accessories to a firearm butt stock not having a fixed accessory mounting rail.