the guidance system used by the Germans in 1942 in the V–2 missile can be considered to be the first use of inertial navigation. It is true that Foucault defined the gyroscope in 1852 and that Schuler developed the gyrocompass in 1908, but the former device was only a measuring instrument and the latter, although of inertial quality, was only a partial inertial system. The Sperry flight instruments of the late 1920's and early 1930's were attitude–indicating not velocity or position-indicating devices. Earnest development of inertial navigation systems began in the United States in the late 1940's and early 1950's by the M.I.T. Instrumentation Laboratory, Northrop and Autonetics under Air Force sponsorship. This work led to the inertial guidance systems for ballistic missiles—both land and ship launched. The 1960's brought the Space Age and the advance of inertial guidance in Apollo. During this time inertial guidance systems also found their way into military and then commercial airplanes. Behind the system development was the simultaneous and necessary development of theory, analysis, components, subsystems and testing. The author, whose professional career has been simultaneous with the growth of inertial navigation, draws on his personal experiences in the field of direct association with many of the people and events involved.