[en] Nations of western Europe, Japan, the Soviet Union, and the United States are working together to demonstrate the practicality of fusion power early in the 21st century. Many difficult engineering problems make fusion development one of the most formidable scientific and technological challenges ever attempted. However, the outlook is promising for achieving an inexhaustible energy source that is safe, economic, and with acceptable environmental effects. The United States magnetic fusion power development program aims at producing fusion energy experimentally in the early 1980's and demonstrating power production on a commercial scale before 2000. This prognosis reflects the confidence gained in scientific successes of the late 1960's through the present. However, many physics problems remain to be solved and many complex engineering problems without obvious solutions await attention. In response to experimental successes and the perceived importance of the fusion energy alternative, the United States effort has grown rapidly. Scientific investigations of plasma physics continue while planned engineering studies lead toward the practical goal of a commercial technology that will take a prominent place among available energy sources of the next century. Development of laser and electron beam fusion proceeds. Alternative fusion devices are investigated for their potential feasibility while the tokamak configuration is used for principal experimental devices. A national program plan and budget coordinates the efforts of federal laboratories, universities and industry. The utilities industry conducts an independent program which is increasingly coordinated with government-sponsored activity. Fusion energy programs of several nations benefit one another and should cooperate more closely in specific problem areas. Achievement of practical fusion power could be advanced through more effective mutually supporting fusion development programs. The economic and technical potentials of fusion power in relation to nuclear fission, solar and other future energy sources can be assessed in general terms. The probability of success in fusion development, while not susceptible to measurement, continues to improve. Fusion can be expected to play an increasingly important role in energy supply world-wide in the early decades of the 21st century. If a commercial scale demonstration reactor (greater than or equal to 500 MWe) operates successfully by 2000, it is reasonable to anticipate as many as 20 to 100 large (1000 MWe) plants by 2020 and an increasing percentage of fusion electrical generating stations thereafter