Abstract Over the past two decades a great many heat-flow measurements and estimates have been made in the southwestern United States and neighboring areas of northern Chihuahua, Mexico. Although these data have enhanced our understanding of the geothermal regime in the southwestern United States, many questions have developed while analyzing the data. In this chapter we present some of the conclusions that can be made from the heat-flow data in the study areas. In addition, we discuss some of the important unanswered questions concerning the geothermal regime in various geologic provinces of the southwestern United States and northern Chihuahua, Mexico. We consider below the heat-flow data in the various geologic provinces of the study area, and when appropriate, interrelate observations from different geologic regions. Articles by Roy and others (1968), Lachenbruch and Sass (1977), and Blackwell (1978) review principles and concepts concerning heat flow in the western United States. The Colorado Plateau is a roughly circular region, about 700 km in diameter, that encompasses large areas in the four-corner states of Arizona, Colorado, New Mexico, and Utah (Fig. 1); it is a relatively stable region within the tectonically and volcanically active western United States. The region has been uplifted 1,000 to 1,500 m since Eocene time, but in general has maintained more lithologic and structural continuity than other areas of the western United States (King, 1959; Eardley, 1962). In keeping with its relatively stable tectonic setting, the heat flow in the Colorado Plateau is intermediate between higher values characteristic of neighboring