Novel hydraulic actuators and hydraulic setting assemblies are provided for use in downhole, oil and gas well tools. The novel hydraulic actuators include a cylindrical mandrel and an annular stationary sealing member connected to the mandrel. A hydraulic cylinder is slidably supported on the mandrel and stationary sealing member and is releasably fixed in position on the mandrel. The stationary sealing member divides the interior of the cylinder into a bottom hydraulic chamber and a top hydraulic chamber. An inlet port provides fluid communication into the bottom hydraulic chamber, and an outlet port provides fluid communication into the top hydraulic chamber. A balance piston is slidably supported within the top hydraulic chamber of the actuator. The piston includes an axially extending passageway. Fluid communication through the piston and between its upper and lower sides is controlled by a normally shut valve in the passageway. In the absence of relative movement between the mandrel and cylinder, the balance piston is able to slide in response to a difference in hydrostatic pressure between the outlet port, is which is on one side of the piston, and the portion of the top hydraulic chamber that is on the bottom side of the piston.