Relativistic hydrodynamics is essential to our current understanding of nucleus-nucleus collisions at ultrarelativistic energies (current experiments at the Relativistic Heavy Ion Collider, forthcoming experiments at the CERN Large Hadron Collider). This is an introduction to relativistic hydrodynamics for graduate students. It includes a detailed derivation of the equations and a description of the hydrodynamical evolution of a heavy-ion collision. Some knowledge of thermodynamics and special relativity is assumed. (Contains 6 figures and 7 footnotes.)