The HERA CO-Line Extragalactic Survey (HERACLES) is a Large Program that used the IRAM 30-m telescope to map CO emission from 48 nearby galaxies. As the second most common molecule, CO acts as our primary tracer of molecular, star-forming gas in galaxies. HERACLES covered a wide area in each target (out to the `optical’ radius, r25) with good sensitivity. We built HERACLES to complement THINGS, SINGS, and associated surveys. As a result we know the detailed distributions of atomic gas (HI), infrared, ultraviolet, and optical light across each galaxy. This makes HERACLES a powerful data set to study the formation of stars from molecular gas, the assembly of molecular clouds out of atomic gas, the distribution of matter (dark, light, stellar, and gaseous) in galaxies, and the dynamics and structure of the interstellar medium.