From the Nobel Prize-winning physicist P. J. E. Peebles teaches the often counterintuitive physics of quantum mechanics by working through detailed applications of general ideas. A principal example used in the book is the hyperfine structure of atomic hydrogen (the 21 cm line): the computation of the energy splitting and the induced and spontaneous transition rates. Peebles makes room for such calculations by omitting unneeded elements that can be readily found in the standard treatises after one fully understands the principles of quantum mechanics. To give a flavor of the discovery of the remarkable world picture of quantum mechanics, the author presents a set of examples of physics that are well worth knowing even aside from their historical interest. Then the general principles of quantum mechanics are stated first in terms of wave mechanics and then in the standard abstract linear space formalism. Measurement theory, an essential part of quantum mechanics, is discussed in some detail. The book also emphasizes the art of numerical estimates. And, lastly, a large number of problems are presented, some easy, some challenging, but all selected because they are physically interesting. The book is designed for advanced undergraduates or beginning graduate students in physics.