A complete overview of quantum mechanics, covering essential concepts and results, theoretical foundations, and applications. This undergraduate textbook offers a comprehensive overview of quantum mechanics, beginning with essential concepts and results, proceeding through the theoretical foundations that provide the field's conceptual framework, and concluding with the tools and applications students will need for advanced studies and for research. Drawn from lectures created for MIT undergraduates and for the popular MITx online course, "Mastering Quantum Mechanics," the text presents the material in a modern and approachable manner while still including the traditional topics necessary for a well-rounded understanding of the subject. As the book progresses, the treatment gradually increases in difficulty, matching students' increasingly sophisticated understanding of the material. . Part 1 covers states and probability amplitudes, the Schr dinger equation, energy eigenstates of particles in potentials, the hydrogen atom, and spin one-half particles . Part 2 covers mathematical tools, the pictures of quantum mechanics and the axioms of quantum mechanics, entanglement and tensor products, angular momentum, and identical particles. . Part 3 introduces tools and techniques that help students master the theoretical concepts with a focus on approximation methods. . 236 exercises and 286 end-of-chapter problems . 248 figures