The thinking behind Agile methodologies isn't hard to find. Indeed, much Agile thinking is simply common sense or good management practice. In the pursuit of engineering rigour, modern management thinking had sometimes been overlooked.
The first thing that the lightweight methodologies did was to simplify the development activities. The previous generation of methodologies had tackled complexity with complexity, so simplicity was high on the list of influences.
Agile development has also been influenced by the arguments of Phil Crosby (Quality is Free) and W. Edwards Deming (see the topic box). Agile developers seek to find faults early in the development cycle. In doing so, costly and disruptive re-work can be eliminated from the later stages of development.
 See Crosby (1980).
The next aspect to be embraced concerned the dirty little secret of software engineering: people. While architecture, engineering, process, notation and tools tend to dominate the literature and teaching of software engineering, there has always been an understanding that it is people who make the real difference. Even by the 1960s, it was apparent that some programmers were simply far more productive than others.
Authors such as Fred Brooks, Tom DeMarco, Timothy Lister and Gerald Weinberg have long written about how to get the most from programmers and teams. Most of this writing parallels similar ideas in common management literature, such as the work of Peter Drucker. ...