Accreting neutron stars and black holes are ideal systems to probe some extreme aspects of physics, which cannot be studied in terrestrial laboratories. X-ray observations of these extreme worlds can provide unique information about these systems. I will give a brief introduction to astronomy, particularly X-ray astronomy, and then I will describe how accreting neutron stars and black holes can be useful to study some important problems of physics and astronomy. Finally, I will talk on AstroSat, the first dedicated Indian astronomy space mission.
