An early, and evolving, paper on emerging agile methodologies was written by MartinFowler.

http://martinfowler.com/articles/newMethodology.html

''from the site ...''

In the past few years there's been a rapidly growing interest in "lightweight" methodologies. Alternatively characterized as an antidote to bureaucracy or a license to hack they've stirred up interest all over the software landscape. In this essay I explore the reasons for lightweight methods, focusing not so much on their weight but on their adaptive nature and their people-first orientation. I also give a summary and references to the processes in this school and consider the factors that should influence your choice of whether to go down this newly trodden path. 

An abridged version was published in Software Development magazine in December 2000 under the title of "Put Your Process on a Diet".

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An excellent article by MartinFowler.  -- AdamLi

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The best, and updated description I know concerning agile processes. -- AstridLumbye

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* CommentOnNewMethodology by StephenGryphon

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(See also AgileProcesses)