Background: The National Evidence-Based Practices (EBPs) Project developed and tested a model for facilitating the implementation of five psychosocial EBPs for adults with severe mental illness in the United States. Methods: The implementation model was tested in 53 sites in 8 states. In each site, one of the five EBPs was adopted for implementation and then studied for a 2-year period using a combination of qualitative and quantitative methods. Findings: At baseline, none of the sites had programs attaining high fidelity. Four factors were identified as influencing fidelity: (a) EBP-specific factors, (b) governmental factors, (c) leadership factors, and (d) fidelity review factors. Conclusion: A multipronged implementation strategy was effective in achieving high fidelity in over half of the sites seeking to implement a new EBP. (Contains 1 table and 1 figure.)