Through-Tubing Rotary Drilling (TTRD) is a slim-hole sidetracking technique successfully employed from North Sea platforms to economically access marginal targets in mature fields from existing wellbores. TTRD typically uses 2-7/8'' standard production tubing with high-torque threaded connections, run from the standard platform drilling rig, to drill-out through the existing completion via whipstock. TTRD sidetracks in the North Sea have been less than 3500 feet in length, and completed with 2-7/8'' or 3-1/2'' flush-jointed liners. As a contingency, the 2-7/8'' drillstring can be cemented in place as a liner. Since (1) TTRD does not require the mobilization of specialized equipment, and (2) the drillstring may be rotated to reduce differential sticking and improve cuttings returns, and (3) the drillstring will tolerate high overpulls, TTRD has proven consistently more economical than Coiled Tubing Drilling (CTD) in similar applications. The objective of this paper is to review