The control rod drive mechanisms installed on the Argonne Experimental Boiling Water Reactor (EBWR) are of an externally operated lead screw and nut type in which the control rod extension shafts are driven through pressure breakdown, collected leakage seals. The mechanisms which are located below the reactor are fabricated from conventional industrial materials, such as carbon steel, brass, cast iron, nylon, etc., and are lubricated with grease. They may be serviced during reactor operation, and are removable from the reactor without the necessity of draining the reactor vessel. This feature has proven to be worthwhile in operation. The mechanism is adaptable to reactors operating at pressures up to 1500 psi and requiring up to a 6-foot control rod stroke. This paper describes the development of the mechanism, the final design and proof-testing program, and some of the particular design difficulties encountered. Some of the other drive types, including electromagnetic jack, hydraulic, and rotary seal rack-and-pinion, considered for EBWR application, are also discussed.