Abstract A black hole is an object that is inside its event horizon at r=r*: a one-way surface that particles and light can only traverse inward. This chapter studies the spherically symmetric nonrotating Schwarzschild black hole. The metric elements change their respective signs when crossing from r>r∗ to r<r*, leading to a role change between space and time. A sufficiently large collection of matter can collapse gravitationally into a (Kerr) black hole. The physical reality of, and observational evidence for, black holes are discussed. There is a correspondence between the laws of black hole physics and those of thermodynamics. In particular, surface gravity at the event horizon behaves like temperature, and horizon area like entropy. This connection is strengthened with the discovery of Hawking radiation, resulting from quantum fluctuations around the event horizon, as particles falling into a black hole can have negative energy.