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A 3 ¼" ultra slim rotary steerable system (US-RSS) has been developed for use in through tubing rotary drilling (TTRD), with the objective of extending the production profile of offshore maturing fields. The introduction of RSS increases the capability of enhanced reservoir development by extending the reach of TTRD, improving drilling performance, increasing production from precise directional control, and lowering overall drilling costs. This development project was a collaborative effort involving several major operators to partially fund production of this ultra slim RSS that could pass through 5" completion tubing. This paper provides an overview of the RSS tool development including system testing at a full scale rig. This testing clearly demonstrated the directional functionality as well as successful communication between the measurement tools and RSS. The testing also allowed design iterations to be made to the drill bit so that the stability, side cutting and hydraulics were optimized and specifically matched to the operating mechanism of the tool. Due to the success in testing, the RSS and bit was taken forward into field trial. Several runs were undertaken with the US-RSS for Dubai Petroleum. In an initial application, the assembly was utilized to drill a dual lateral oil producer / injector that traditionally would have been cost prohibitive due to time, or impossible due to torque and drag considerations. This lateral was achieved, including successful completion of the first ever ultra slim open-hole sidetrack. The objective of a recent application was to drill the horizontal drain of a re-entry well. Use of the US-RSS delivered a saving of 6 days in time and approximately $1.8million in cost. The assembly also achieved a world record for footage with a US-RSS, drilling over 2000' with ROP 30% higher than the average produced with downhole motors. Introduction The US-RSS is an essential tool in the quest to drill sub-5 ½ in. wells with complex profiles so commonly required in the crowded subsurface environment of a modern, mature field. A US-RSS enables significantly improved performance and flexibility over more standard slide drilling options by improving hole quality, ECD and weight transfer even in very high Dog Leg Severity (DLS) and long step out well plans. This was a critical component of the successful operations to drill the long step out drains in the application for Dubai Petroleum. The Advantages of Ultra Slim RSS are numerous;The control of hole quality and cuttings management is essential to effective Ultra Slim well development as this enables targets further away from the mother bore to be reached before the traditional limitations of ECD and stand pipe pressures limit the step out possible. A fully rotating RSS system will improve weight transfer and hole quality while reducing or preventing the cutting beds often seen to build up during slide drillingA high dog leg capability allows more complex well plans to be achieved and also enables kick off lower down in the reservoir allowing deeper, shorter sidetracks