Web 2.0 tools offer new possibilities for teaching and learning. PrimaryAccess is a Web 2.0 tool designed for K-12 history education. PrimaryAccess shares many of the characteristics of other Web 2.0 applications, but its educational focus makes it different from generic Web applications. Our work developing and researching PrimaryAccess has helped identify the tensions and opportunities for integrating Web 2.0 tools into K-12 instruction. Web 2.0 tool development and use are best explored in a disciplinary context. Mishra and Koehler's construct of Technological Pedagogical Content Knowledge (TPACK) provides a useful framework for this research and development process. (Contains 1 table.)