Usability expresses how well a computerised system supports human activity. The human-computer interaction occurs via the user interface, which is constituted by a Graphical User Interface (GUI) to a great extent. The issue is to integrate GUI design and usability engineering with traditional system development in order to slide on the man-machine scale. The purpose of writing this thesis was to find out how to approach GUI design with respect to stakeholders? interests, technical issues, functional requirements and budget limitations. After having conducted four system development projects of varying nature, the outcome was a usability toolbox with techniques, tools and ideas for all situations. The usability toolbox offers inspiration and guidance for your interactive and creative moments and can support GUI design in any phase of a system?s life cycle, in any project.