Andrew Fabian has made a remarkable variety of discoveries in X-ray astronomy. He has been especially influential because of his energetic success in making complementary observations with ground-based telescopes, and because his broad programmes are all motivated by deep theoretical understanding. His early advocacy of ‘cooling flows’ in clusters of galaxies — and their importance for galaxy formation, cluster evolution and dark matter — has been vindicated by the remarkable results from the ROSAT and ASCA satellites. He has investigated the spectra and variability of galactic nuclei and has, ever since his pioneering experiments using sounding rockets, been an authority on the cosmic X-ray background. With his research group, he has been responsible for much of the most thorough modelling of all classes of X-ray sources. As a theorist, he has also contributed original (and often durable) ideas on compact objects, binary stars, jets and cosmology. He has been an international leader in X-ray astronomy, involved in major worldwide collaborations, and prominently engaged in almost all current and future missions.