The Internet is widely used in K-12 schools. Yet teachers are not well prepared to teach with the Internet, and its use is limited in scope and substance. This article uses case studies of three high school science teachers to develop a framework for teaching with the Internet, exploring how the Internet shapes and is shaped by classroom practices. The framework includes five affordances of resources: (a) boundaries, (b) authority, (c) stability, (d) pedagogical context, and (e) disciplinary context. These interact with fundamental challenges of teaching to produce wide variation in Internet use. The case studies suggest that affordances vary because of activity design and characteristics of the resource. Challenges to teachers depend on how they position themselves with respect to the affordances. (Contains 4 tables and 4 footnotes.)