Background: Local implementation of evidence-based curricula, including sex education, has received increasing attention. Although there are expectations that practitioners will implement evidence-based programs with fidelity, little is known regarding the experiences of instructors in meeting such standards. During 2005 to 2009, the California Department of Public Health funded local agencies through its Teen Pregnancy Prevention Programs (TPP) to provide comprehensive sex education. Methods: To improve understanding of how agencies implemented curricula, in-depth telephone interviews with 128 coordinators were conducted in 2008 to 2009. Qualitative data were analyzed for content and themes. Selected data were quantified and analyzed to examine differences in curriculum adaptations across settings and curricula type. Results: Whereas over half of the TPP agencies (59%) implemented evidence-based curricula, most agencies (95%) reported adapting the curriculum, with the majority (83%) adding content. Reasons for adaptations included ensuring that the material was accurate and appropriate; responding to logistical or time constraints; and other factors, such as parental and institutional support. Conclusion: These adaptations reflected agencies' efforts to balance state and local requirements, maintain curriculum fidelity, and provide more up-to-date and accessible information. These experiences highlight the need for guidelines that enable appropriate adaptations, while maintaining fidelity to the core components of the original curriculum.