We not only ask what makes Earth habitable (e.g., suitable climate, chemical composition, presence of water), but also how does our global environment maintain habitability? What makes our planet sustainable? In particular, how does life on this planet affect and regulate the chemical environment that in turn allows life to continue and not perish? Understanding the biological and chemical processes that regulate the environment is essential to answering this question. And answering this question will help us understand how humans have been altering the chemistry of the world, with ramifications for other life, present and future. This is the essence of biogeochemistry. In this class, we will explore the imprint that biota (including humans) have on the chemistry of the atmosphere, oceans, and lithosphere. And we will learn HOW we can measure key biogeochemical processes, which may be largely invisible to the human eye, but are critically important in controlling exchange of chemicals between different reservoirs.