The first edition of this book was based on my lecture notes for an upper-levelundergraduate cosmology course at The Ohio State University. The students tak-ing the course were primarily juniors and seniors majoring in physics and astron-omy. In my lectures, I assumed that my students, having triumphantly survivedfreshman and sophomore physics, had a basic understanding of electrodynamics,statistical mechanics, classical dynamics, and quantum physics. As far as math-ematics was concerned, I assumed that, like modern major-generals, they werevery good at integral and differential calculus. Readers of this book are assumedto have a similar background in physics and mathematics. In particular, no priorknowledge of general relativity is assumed; the (relatively) small amounts ofgeneral relativity needed to understand basic cosmology are introduced as needed.The second edition that you are reading now is updated with observationaland theoretical developments during the 14 years that have elapsed since the firstedition. It has been improved by many comments by readers. (My thanks go to theeagle-eyed readers who caught the typographical errors that snuck into the firstedition.) The second edition also contains an extended discussion of structureformation in the final two chapters. For a brief course on cosmology, the first tenchapters can stand on their own.