The Phosphorus Factbook provides a systemic, data-based analysis of the future availability of mineral and naturally weathered phosphorus. We consider, analyze, and forecast phosphorus availability for the future on three scales: the short term (i.e., < 10 years), the midterm (i.e., 10–100 years), and the long term (i.e., > 100 years). This chapter presents data on the lifetime, current reserves, and resources of mineral phosphorus as well as its occurrence in soil compartments. A major goal of The Phosphorus Factbook is to understand how long humankind will have mineral phosphorus available. Because knowledge on the topic is incomplete, we identify six gaps in knowledge as desiderata that should be the subjects of future research. The conceptual framework for modeling the dynamics of phosphorus supply and demand is the Feedback Control Cycle for mineral resources, and as the term “Factbook” suggests, the text provides basic data on those processes.