The quest for obtaining and controlling an extremely low gas density environment, typically classified by the vacuum science community as extreme high vacuum ( < 10-10 Pa), has been a journey spanning more than a half century. Unlike other quests for pushing a parameter on the scientific frontier, the push for the extreme high vacuum environment has not been driven simply by the desire to claim a new record breaking parameter. Rather, the need for an extreme high vacuum environment has come primarily from scientific and a technical applications which have driven the march to higher vacuum environments. This paper will review the interplay of applications that have required extreme high vacuum and the technology needed to produce, sustain and characterize the environment.