Military electronics and sensors with hermetic packages specified to 125°C have been the mainstay of the industry for many years. Oilfield service companies have long designed these components into downhole tools that operate in environments up to 175°C, but for significantly reduced periods of time. However, new development of hermetic military electronics has all but ceased, forced from the marketplace by less demanding, lower temperature consumer applications such as cell phones, portable computers, and personal data assistants (PDAs). Unfortunately, downhole tools designed with consumer electronics face substantially reduced life at higher temperatures. Therefore, oilfield service companies must employ innovative solutions and rigorous environmental qualification to ensure maximum reliability. At the same time the desire for additional services in ever more extreme environments is growing. This paper describes the development program, tool architecture, mission profile, and prototype testing results to date of a retrievable, reseatable measurement-while-drilling (MWD) tool specifically designed to operate at temperatures up to 200°C and pressures up to 35,000 psi.