Together, the coupled transfer of carbon and nitrogen through the atmosphere, organisms, soils, and water drive key ecosystem functions and will play central roles in determining how the biosphere responds to climatic change. Understanding the patterns, players, and processes of terrestrial carbon and nitrogen cycling requires comprehensive knowledge of terrestrial ecosystems from the molecular to global scales and pulls from experimental, observational, and theoretical research approaches. Only through this understanding of coupled carbon and nitrogen cycling can we accurately model and predict future biosphere responses to global change.