Abstract Reliable, low cost coiled tubing window milling technology has become the cornerstone of ARCO Alaska's coiled tubing sidetrack program at Prudhoe Bay. ARCO began to investigate coiled tubing drilling technology in 1989. The program grew into an aggressive field application of this technology, with over thirty wells drilled in the last 2 1/2 years. The program's first window was cut below production tubing with a coiled tubing bottom hole assembly (BHA) in the fall of 1994. To date, 14 wells have been sidetracked using this technique. The windows are cut off of a specifically formulated cement plug without the use of a whipstock. Reliability, always high, has continued to improve, while the cost and time required continue to come down. ARCO has been able to reduce sidetrack costs by an average of 40% compared to conventional sidetrack methods through the continued evolution of coiled tubing drilling technology, of which window milling is a part1. This window milling technique should also have the potential to save time and money in slim hole and conventional drilling applications as well.