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The architecture is developed in early iterations of the very first lifecycle of a new system. In the Unified Process it is primarily done during the elaboration phase. Actually we can define the elaboration phase as the one in which you need to get a stable architecture. If you have not got a stable architecture you are not yet done with the elaboration phase. Having a stable architecture is a prerequisite to making a good plan – a plan with high credibility--to guide the construction phase.

In the elaboration phase you carry out each iteration in the usual way, that is, you start with requirements capture and follow with analysis, design, implementation, and test. In this phase, however, you focus particularly on the architecturally significant use cases and their related requirements. As always, the use cases drive your work in every iteration. The result is now an implementation of a small portion of the ultimate system, what we might call a skeleton of the system with a few software "muscles." More formally, we call it the italics architectural baseline. This baseline is more than a set of extracts from the models; it actually runs--to a degree.