Population biology has been investigated quantitatively for many decades, resulting in a rich body of scientific literature. Ecologists often avoid this literature, put off by its apparently formidable mathematics. This textbook provides an introduction to the biology and ecology of populations by emphasizing the roles of simple mathematical models in explaining the growth and behavior of populations. This new edition has three significant extras compared to the successful first edition: first, it includes more problems and many of these now include answers. Second, it includes a series of spreadsheets written in Microsoft Excel to illustrate most of the models in the book. Third, the number of biological examples has been increased. The author assumes acquaintance with only elementary calculus, and provides tutorial explanations where needed to enhance the book's value to students in classes ranging from population biology and population ecology to mathematical biology and mathematical ecology.