Star formation differs widely in different galaxy types (e.g., Kennicutt 1998), ranging from slow, low-efficiency events that may be long-lasting to intense, short-duration starbursts. Along the Hubble sequence, typical global present-day star formation rates range from ~ 0 M yr-1 in giant ellipticals to ~ 20 M yr-1 in gas-rich, late-type spirals. Starburst galaxies show star formation rates of up to ~ 100 M yr-1, and ultra-luminous infrared galaxies (ULIRGs) appear to form up to ~ 1000 M yr-1 in stars. Star formation may be localized or encompass a large fraction of the baryonic gas mass of a galaxy. It may be continuous, declining or increasing in intensity, or episodic. It may be triggered by internal processes within a galaxy or by interactions with other galaxies. Typical sites of present-day star formation in galaxies are located in the extended disks of spirals and irregulars, in the dense gas disks in galaxy centers (circumnuclear star formation), and in regions of compressed gas in starbursts, interacting galaxies, or tidal tails. The star formation histories of galaxies vary with galaxy type, mass, gas content, and environment as well as with time, and are tightly coupled with their chemical evolution.