Abstract This is a summary of state of the art horizontal well technology and a review of the economic benefits of horizontal wells. The paper describes various reservoir applications of horizontal wells from primary recovery to EOR applications. The paper includes field examples of different applications. Although horizontal wells have been drilled as early as 1927, the major thrust of drilling horizontal wells started in 1980. Initial wells were short length wells (about 250 ft. long wells). In 1985, the first medium radius horizontal well was drilled using a down-hole mud motor. Since then, using horizontal wells has become a common practice. Today, the medium radius drilling technique is the most commonly used drilling method. In the U.S., the majority of applications are in low permeability, naturally fractured, carbonate reservoirs. However, in California, Alaska and Gulf of Mexico most of the wells are drilled in clastic reservoirs. Similarly, outside the U.S., most of the horizontal wells are drilled in clastic reservoirs. Horizontal wells have been used to produce thin zones, fractured reservoirs, formations with water and gas coning problems, waterflooding, heavy oil reservoirs, gas reservoirs, and in EOR methods such as thermal and CO2 flooding. The paper includes field examples with cost benefit analysis for various applications.