A Build Process is a set of (hopefully automated) steps to convert your software from source to deliverables.

Having a BuildProcess fully automated is in general a BestPractice, but it's absolutely vital when you're doing ContinuousIntegration (in XP or otherwise).

* Not only fully automated, but available to anyone.  All developers should be able to (re)build the system, at any time.

-- KyleCordes
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See also: UsingNmake, AntHill, CruiseControl, IntegrationGuard, ApacheAnt, NantTool