The growing complexity of systems we deal with lead to a need for a new way of analysing the needs of users. For example, urban planning can be coupled with a process of co-designing with residents and other users of a space.
One interesting model for a design process is taken from the Interaction Design Sketchbook by Bill Verplank, it postulates methods of approaching a design problem from either an idea, a metaphor, model, display, error, scenario,task or control. All of these are eventually played through, but they offer differing conclusions.
One central concept is the differentiation of path knowledge and map knowledge. Path knowledge represents intuitive, simple causes and effects. If one presses a button and a light goes on, this can be recognized purely with our senses.
Map knowledge is more relevant in more complicated systems such as UI or OS designs. The user learns a map of the system, and can get to a goal using their own representation.
This also brings us to a useful paradigm of how a computer is perceived. It can take the form of a brain, a tool, or as media, depending on the use, and how the interaction is structured.
Interaction Design Methods 