This book provides a comprehensive account of the ideas underlying modern cosmology. Assuming no previous knowledge of astronomy or cosmology, it takes readers beyond introductory texts to the point where they are able to read and appreciate the scientific literature, which is broadly referenced in the book, and includes problems to build practical knowledge. The authors present the standard big bang universe and introduce current inflationary cosmology, emphasizing the underlying physics without excessive technical detail. Cosmological models are treated without reliance on prior knowledge of general relativity so the necessary physics are introduced in the text as required.