The industry recognizes its increasing need for extended-reach drilling (ERD) and horizontal drilling to improve productivity and reduce cost. In 1991,the authors and a few others forecast that steerable rotary drilling (SRD) would become one of the critical technologies needed to facilitate further extended reach. In 1992, we began devising an SRD system. Its bias unit carries hydraulic actuators that displace the drill bit relative to the hole just drilled and, hence, deviate the new hole as required. This bias unit rotates with the bit. Nonrotating bias is obtained simply by synchronous modulation of its rotating bias. A control valve within the bias unit is mechanically linked to a servo-orientated control unit. The basic concept was proved by drilling an experimental well during 1994.