Usability and user interfaces are essential for most programs. Therefore, good us- ability is an obvious goal, but how does one achieve good usability? Who is the actual user of the program for whom the usability is optimized for? What is the most efficient way to start designing usability of a program, what kind of tools and methods there are for this purpose? And how does one measure usability effectively, reliably and comparably? The client for this Master’s Thesis was the software company ATR Soft Oy, which provides various services, such as integrations, software development and software project management. The subject of this thesis was to design a new, as user-friendly user interface as possible for the web version of their product, the CT Publisher. The principles and methods of user centered design were used while doing the design work. In the beginning of this thesis we analyze what user-centered design means and how it is done. In the case section this knowledge is applied in an the chosen design project. The results confirmed the efficiency of the methods presented in the theoretical part of the thesis and many of the tools used in user-centered design were proven useful. As a conclusion, we note that especially user personas, user interface prototypes and usability testing with users are very efficient methods in guaranteeing good usability.