Background: The implementation and integration of computer technologies in K-12 education has seen nearly constant growth since the early 1980s (Culp, Honey, Mandinach & Bailey, 2003), in part because this trend has become synonymous with skills that students will need as participants in a competitive global economy (Culp et al., 2003). It has been argued that the integration of Web 2.0 tools into K-12 education will help students acquire such skills, as the web-based platforms offered by Web 2.0 provide an open, dynamic environment allowing all end-users to participate, interact, and collaborate with instructors, peers, friends, and unknown people worldwide (Buffington, 2008; Jonassen, Howland, Marra & Crismond, 2008; Solomon & Schrum, 2007). This paper investigated the relationship between in-service teachers' self-efficacy and the integration of Web 2.0 tools (e.g., blogs, wikis, podcasts, social networking sites, image/photo sharing sites, & course management systems) at K-12 public schools in the United States. Goals: This study identified the factors predicting the utilization of these Web 2.0 tools in classroom instruction. It provides insight into the barriers of technology integration for future implementation. Research Method: A nationwide stratified sample frame was utilized to collect quantitative data through a web survey. A multiple regression analysis was employed to isolate the factors influencing the integration of Web 2.0 tools in K-12 classrooms. Results: A total of 559 in-service teachers responded to this research invitation. The results revealed public in-service teachers reported a low level of self-efficacy in using Web 2.0 tools, as well as a low frequency of Web 2.0 tools integration in their classrooms. Three out of five predictors included: teachers' self-efficacy, professional development, and school administrative support significantly predict the use of Web 2.0 tools. (Contains 2 tables.)