This book provides a clear and logical path to understanding what quantum mechanics is about. It will be accessible to undergraduates with minimal mathematical preparation: all that is required is an open mind, a little algebra, and a first course in undergraduate physics. Quantum mechanics is arguably the most successful physical theory. It makes predictions of incredible accuracy. It provides the structure underlying all of our electronic technology, and much of our mastery over materials. But compared with Newtonian mechanics, or even relativity, its teachings seem obscure--they have no counterpart in everyday experience, and they sometimes contradict our simplest notions of how the world works. A full understanding of the theory requires prior mastery of very advanced mathematics. This book aims at a different goal: to teach the reader, step by step, how the theory came to be and what, fundamentally, it is about. Most students learn physics by learning techniques and formulas. This is especially true in a field like quantum mechanics, whose content often contradicts our common sense, and where it's tempting to retreat into mathematical formalism. This book goes behind the formalism to explain in direct language the conceptual content and foundations of quantum mechanics: the experiments that forced physicists to construct such a strange theory, and the essential elements of its strangeness.