Since the discovery of the growth substance activity of the phenoxy acids by Zimmerman and Hitchcock (14) in 1942 and the announcement of 2,4–dichlorophenoxyacetic acid (2,4–D) and 2,4,5–trichlorophenoxyacetic acid (2,4,5–T) as herbicides by Hamner and Tukey (9) in 1944, a new era in chemical weed control has developed. In 1949 workers at the Texas Agricultural Experiment Station at Spur, and the Weed Investigations Section of the U.S.D.A. (6) reported that aerial application of 2,4,5–T offered much promise of effectively and economically controlling mesquite on rangeland. Since that time, approximately 1,500,000 acres of mesquite infested grassland have been treated with this compound by aerial application. Though widespread use of 2,4,5–T has been made, the factors that influence its effectiveness in killing mesquite are not well understood. This paper attempts to summarize the information that is available on some of these factors.