We implemented Agile (set of SCRUM - management and XP - engineering practice) in an environment that was waterfall with large projects in an environment that was heavily integrated. The waterfall police were everywhere. As you can imagine, many projects failed. Having done Agile at a previous employer, we received permission to trial agile for the project.
Internal to the team, we used the Agile practice. Externally, we wrapped the agile practices with waterfall processes meaning primarily reporting. Thus, we looked from the outside like a waterfall project. However, there was a big difference, internally we were using agile and consequently we delivered, on time, within budget with high quality.
The critical success factors were embedded coaches (Iteration Manager Coach, Dev Lead Coach, Test Lead Coach and a Solution Analyst Coach). Securing commitment from dependent system in advance (required that we look ahead to identify depend systems and the work required from those systems) was a must in a heavily integrated environment. Prior to starting, we immersed the technical and business members of the team in an agile boot camp. This ensured that the key players (product owner and technical team) knew there roles and could execute effectively. Finally, the wrapping of the project with waterfall reporting enabled us to tie into all the existing reporting structure in the enterprise.
The net result is that the company is now moving waterfall projects to agile. This is all possible only because we have been able to deliver high quality software at a sustainable pace.