BIOLOGY is an authoritative majors textbook with evolution as a unifying theme. In revising the text, McGraw-Hill has consulted extensively with previous users, noted experts and professors in the field. It is distinguished from other texts by its strong emphasis on natural selection and the evolutionary process that explains biodiversity. Not only has the book been thoroughly updated to reflect rapid advances, there is more emphasis today on the teaching of concepts and this has led to significant changes in how the material is presented. Technology also plays a greater role in teaching and the Online Learning Center found at http://www.mhhe.com/raven6 provides professors and students alike with an abundance of resources. Five considerations influenced this revision. They are: 1) Focus on concepts; 2) Reinforcing Ideas; 3) Emphasizing relevance to students; 4) Keeping up with new developments; and 5) Careful editing.