GUI Layout And Flow Defines the set of screen layouts used for the GUI, and the flow of working with these screens (what happens when the user clicks each button, which screen follows which other screen, etc). Must match the functional requirements, not alter them. Tends to cause the most grief (everyone "knows" how a good GUI looks, and hence requests of "move the 'Details...' button 2 pixels up" come in endlessly. To overcome this problem - a very strict deadline for the design should be defined.