New from The Biology Project of the University of Arizona, this online tutorial introduces the dynamic complexes of proteins, carbohydrates, and lipids that comprise cell membranes, and relates how membranes are important for regulating ion and molecular traffic flow between cells. Each section of this Web site takes the form of a multiple choice question. Answer the question correctly, and a brief explanation of each answer choice will be displayed. Answer the question incorrectly, and a short but helpful tutorial with colorful diagrams will help get you on the right track. This would be an valuable Web site for students wishing to test themselves on cell membrane structure and function, but would not be especially useful for those new to the subject.