Historically, biology has not been a heavily quantitative science, but this is changing rapidly (Ewing 2002; Gross 2000; Hastings and palmer 2003; Jungck 2005; Steen 2005). Quantitative approaches now constitute a key tool for modern biologists, yet undergraduate biology courses remain largely qualitative and descriptive. Although biology majors are often required to take a full year of calculus, these courses generally use examples unrelated to biology (Gross 1994) and ignore fields of mathematics that may be more relevant to biology, such as liner algebra or theoretical probability and statistics (NRC 2003). In this series of interactive, web-based modules, students are introduced to the mathematical underpinnings of introductory biology in an informal but powerful way.