Black holes are exotic objects whose gravitational field is so strong that nothing, not even light, can escape from them. Surprisingly, black holes not only swallow matter, but also eject jets of highly energetic particles at nearly the speed of light. What drives such powerful jets? This has been one of the biggest mysteries in astronomy. Recent studies suggest that the black-hole jets have significant impact on galaxy formation and play a key role in the evolution of the universe. My collaborators and I have been observing the emissions from the gas falling onto and ejected from black holes, using X-ray satellites together with ground-based optical and infrared telescopes. Our main goals are to reveal the physical mechanism of the jets and to understand the cosmic history.