Validate
Once the prototyping is complete, it's time for the designer's final evaluation. At this point the designer has to re-access whether the prototype(s) actually fulfill the requirements gathered back in the research stage and determine whether or not the technology for implementing the design is suitable for the project.
Validation is best done using a sample set of real users, and a whole industry has grown up around this one step -- Usability Testing. The most famous of this breed is Jakob Nielsen, who's influence is noted by the great degree to which he's revered and disdained. (often at the same time)
Personally, I think Jakob makes a lot of sensible points, but is often draconian in his views on implementation. Notably his own home page is an example of how over-engineering usability, can lead to a holistic design failure. On the other hand there are an innumerable number of sites on the Internet that overflow in style, but have no substance on which to stand. I think there's a happy medium in there somewhere, and it's the job of a good designer to find it.
