Coiled-tubing (CT) drilling is a rapidly growing new technology that has been used for shallow new wells and re-entry applications. Through-tubing drilling has evolved as a major application for CT drilling. The remaining key enabling technology for viable through-tubing drilling is the ability to sidetrack in casing below the tubing tail. This paper describes the three technologies developed for sidetracking and presents a mathematical model of forces, penetration rates, and torques for window milling with the cement-sidetracking(CS) technique. Window milling has been a "seat of the pants" operation in the past. To our knowledge, this is the First published work on the mechanics of window milling. The results from several yard tests and one field test are presented and show some of the problems associated with sidetracking.