The GUI was the first real breakthrough, it liberated the user’s mind. People no longer had to conform their thinking to the machine’s requirements. Yesterday, it was the limitations of the machine that determined the user's actions. Today, it is the user's psychology that determines the machine’s actions. Interface design has advanced by keeping human factors in mind, especially when doing usability testing. The Graphical User Interface was a massive increase in usability. Function went from something hidden behind the command line that required arcane runes to unlock, to a simple screen graphic.

Interface protocols are the reason why modern applications can perform quite well in the hands of people who have no understanding of the thousands of lines of code that initiate the functions they take for granted. For the majority of users, the interface is the program.

Successful interface design is based on a user centered approach that not only takes the user into account, such as education level and psychology, but also the work environment and even the culture in which the user operates. Modelling the interface to reflect what the user is already accustomed to, such as the famous trash can icon on the desktop, has become standard practice. It is extremely useful to use conceptual models that exist in the real world. This will render the application much easier to comprehend. People are a lot better at recognising things than recalling information, if there is a way to facilitate recognition, then use it.

Computers are an increasingly important part of society. There may well come a day when nothing will function without them. This means that usability will become even more important. And the ultimate interface may one day blur the line between human and machine.