The development of ultrahigh vacuum technology for high-energy particle accelerators and magnetic fusion devices provided essential contributions to the progress in these two scientific endeavors over the last four decades. Storage rings were first proposed as an efficient means of producing intense high-energy particle beams by G. K. O’Neill in 1956. The milestone demonstrations of the importance of this invention came with the operation of the ISR device at CERN in 1971, a 2 km long vacuum vessel which operated at 10(-12) Torr and the SPEAR electron-positron collider at Stanford in 1972. These achievements required development of new materials, materials processing techniques, vacuum components, and vacuum instrumentation. UHV development continues through the present design efforts for the 27 km LHC ring at CERN, and numerous high intensity synchrotron light sources worldwide. A similar progression in UHV development preceded the operation of the present generation of large tokamaks built to demonstrate the scientific feasibility of magnetic fusion energy : The JET device in Europe (U.K.), the JT-60 in Japan, and TFTR at Princeton. Development efforts in UHV compatible first-wall materials, efficient vessel conditioning procedures, and high capacity pumps are essential for the operation of these devices and continue to be active areas of research for die next generation of fusion demonstration devices.