The student of calculus is entitled to ask what calculus is and what it can be used for. This short book provides an answer. The author starts by demonstrating that calculus provides a mathematical tool for the quantitative analysis of a wide range of dynamical phenomena and systems with variable quantities. The text then looks at the origins and intuitive sources of calculus, its fundamental methodology, and its general framework and basic structure, before examining a few typical applications. The author's style is direct and pedagogical. The new student should find that the book provides a clear and strong grounding in this important technique.