The three High-Energy Astrophysics Observatory (HEAO) missions were a series of telescopes launched by NASA to study X-rays, gamma rays, and cosmic rays. During its year and a half of operation, HEAO-1 provided continuous monitoring of many astronomical X-ray sources, such as quasars and pulsars. HEAO-2, also called the Einstein Observatory, operated from November 1978 to April 1981, and took the highest resolution X-ray images of the sky up to that time. HEAO-3 was launched in 1979 and focused on gamma rays and cosmic rays. Einstein helped pave the way for Chandra, NASA’s flagship X-ray observatory mission, while HEAO-3 led to the Compton Gamma Ray Observatory.