Objective: Describe a process for designing, building, and evaluating a theory-driven social media intervention tool to help reduce parental concerns about vaccination. Method: We developed an interactive web-based tool using quantitative and qualitative methods (e.g., survey, focus groups, individual interviews, and usability testing). Results: Survey results suggested that social media may represent an effective intervention tool to help parents make informed decisions about vaccination for their children. Focus groups and interviews revealed four main themes for development of the tool: Parents wanted information describing both benefits and risks of vaccination, transparency of sources of information, moderation of the tool by an expert, and ethnic and racial diversity in the visual display of people. Usability testing showed that parents were satisfied with the usability of the tool but had difficulty with performing some of the informational searches. Based on focus groups, interviews, and usability evaluations, we made additional revisions to the tool's content, design, functionality, and overall look and feel. Conclusion: Engaging parents at all stages of development is critical when designing a tool to address concerns about childhood vaccines. Although this can be both resource- and time-intensive, the redesigned tool is more likely to be accepted and used by parents. Next steps involve a formal evaluation through a randomized trial.