Volume 75 of Reviews in Mineralogy and Geochemistry addresses a range of questions that were articulated in May 2008 at the First Deep Carbon Cycle Workshop in Washington, DC. At that meeting 110 scientists from a dozen countries set forth the state of knowledge about Earth's carbon. They also debated the key opportunities and top objectives facing the community. Subsequent deep carbon meetings in Bejing, China (2010), Novosibirsk, Russia (2011), and Washington, DC (2012), as well as more than a dozen smaller workshops, expanded and refined the DCO's decadal goals. The 20 chapters that follow elaborate on those opportunities and objectives.