Star formation is at the nexus of astrophysics: stars are believed to be responsible for the re-ionization of the universe, they created all the heavy elements, they control the formation and evolution of galaxies and the formation of stars naturally leads to the formation of planets. It is therefore critical to understand the rate at which stars form. Observations of the star formation rate are encapsulated in the Kennicutt-Schmidt relations between the star formation rate and the amount of gas in a galaxy. More recently, spatially resolved measurements of the atomic and molecular gas, the stellar content, and the star formation rate have been obtained for nearby galaxies. Observations have also shown that star formation is inefficient—the fraction of the mass of a molecular cloud that is converted into stars is about 1% per free-fall time in environments as diverse as small, local molecular clouds and starburst galaxies.  The overall features of these observations are naturally explained in the theory of turbulence-regulated star formation together with a determination of the molecular fraction in a multiphase interstellar medium (ISM); this theory applies over a wide range of conditions. Much more precise agreement with observations of nearby disk galaxies can be obtained by determining the star formation rate needed to maintain the multiphase ISM in such galaxies.