This guidebook, Hydrogeology of Trans-Pecos Texas, grew out of our interest in the hydrogeology of desert environments. After we participated in Marty Mifflin and Jay Quade's 1988 field trip through eastern Nevada; we realized that, although there were many similarities, the hydrogeology of Trans-Pecos Texas was quite different from what we had observed in Nevada. Thus the seed was planted for this field trip and guidebook. We hope that those who participate in this trip will likewise become intrigued with the hydrogeology of West Texas and will return to their own hydrologic environments, arid or otherwise, with some new approaches and concepts. The guidebook is a compendium of information on the hydrogeology of Trans-Pecos Texas. It includes a road log for the 2-1/2-day trip, a list of references cited in the road log, six technical papers, and seven papers previously published in scattered symposia and other guidebooks. These papers provide greater detail than can be contained in the road log. Much of the information for the road log was generated during four previous field trips (1984, 1986, 1988, and 1989) to the Trans-Pecos region, conducted in conjunction with the graduate course in Geology and Hydrology offered at the Department of Geological Sciences, The University of Texas at Austin. We appreciate the interest and formal and informal contributions of numerous students. Malcolm Ferris assisted in compiling the second- and third-day road logs. The 1989 Geology and Hydrology class served on the rehearsal field trip. Financial and logistical support of the Department of Geological Sciences, The University of Texas at Austin, for these field excursions is gratefully acknowledged.