The safe landing of STS-121 on July 17, 2006, marked the successful completion of NASA's second Return to Flight (RTF) Mission. This Final Edition of NASA's Implementation Plan for Space Shuttle Return to Flight and Beyond provides an update to NASA's responses to the Columbia Accident Investigation Board recommendations and observations, as well as updates to the Space Shuttle Program (SSP) "raising the bar" initiatives through the two RTF missions (STS-114 and STS-121). Also included is an update to RTF-related costs through the end of Fiscal Year 2006. This Final Edition provides technical updates that reflect the completion and post-flight analysis of STS-121. These include NASA's improvements to ground and on-board ascent and on-orbit imagery capabilities, changes to the External Tank designed to reduce the ascent debris risk, and the growing maturity of NASA's on-orbit thermal protection system (TPS) repair capabilities. These updates describe how swapping out the first STS-114 External Tank, ET-120, led to an opportunity to analyze External Tank foam from a tank that had gone through a complete cryogenic loading cycle. This analysis led to a better understanding of the root cause mechanisms behind foam loss, and specifically resulted in the removal of the protuberance air load (PAL) ramps from the External Tank for STS-121 and all subsequent flights. STS-114 also provided valuable experience and lessons learned for preventing the loose tile gap fillers seen on that flight. Following implementation of improved procedures to validate or replace gap fillers, there were no mission impacts from protruding gap fillers on STS-121. Several new capabilities were also first demonstrated on STS-121. New cameras with improved viewing angles were installed on the Solid Rocket Boosters, providing engineers with better coverage of the vehicle during ascent. The Orbiter Boom Sensor System's ability to serve as a stable work platform for potential TPS repairs was validated. A TPS repair material, non-oxide experimental adhesive (NOAX) was successfully tested by astronauts during a space walk in the cargo bay, demonstrating that NOAX was effective in repairing small gouges and cracks in reinforced carbon-carbon material. Finally, a late inspection of the TPS two days prior to re-entry was added to the mission timeline to inspect the Orbiter for any significant damage due to micrometeorite and orbital debris impacts. Together with the results from STS-114, STS-121 succeeded in demonstrating the effectiveness of many of the safety improvements made to the Space Shuttle system during RTF. NASA will continue to pursue safety improvements to the Space Shuttle system until the planned retirement of the Space Shuttle in 2010. Improvements to the Space Shuttle system from STS-115 (the first International Space Station assembly flight after RTF) onwards will be diligently reviewed and accounted for as part of the normal SSP and Agency requirements and resources review process. As we continue to pursue rigorous engineering analyses across NASA, we are encouraging healthy debate to identify, understand, and eliminate unacceptable risk that remains in the Space Shuttle Program. RTF is not simply a set of objectives or goals, but an evolution and improvement in the way we do business. While we work to safely fly the Space Shuttle, we are also working to efficiently transition those elements that are applicable to our future in exploration and discontinue those elements that will no longer be needed.