Boehm recognised the shortcomings of the Waterfall Process in the early 1980s. His development cycle acknowledged both the need for experimentation and the essential requirement for some sort of development process structure. The solution to these problems was through an iterative development approach. Each iteration of a system's development included a review phase, during which the targets for the next iteration of development would be established. The review phase also, and crucially, permits project managers to establish whether the approach being followed is viable. The risk of a development failing will always exist. It may be through the specification of a system that could not be created, or possibly as a result of a competitor's early arrival in the same market sector. In either case, the system's development path may be altered if an iterative approach is followed, since its inherent flexibility is far greater than that in the Waterfall method.
Boehm's work has proved to be the foundation of radical developments in the production of reliable software systems. His iterative approach has been followed and elaborated by many of the succeeding developments. Follow the link for an outline description of the Unified Process.