Usability indicates the ”absence of frustration”. But what makes a product or a service really usable? The course will introduce to the basic concepts, theories and methods of usability engineering and testing. We will discuss quality attributes that constitute good usability and will identify design flaws and product defects. Special emphasis will be put on quantitative measures to determine the ease-of-use of a system in various stages of development. Students will learn how to set up and run an empirical user study, including (but not limited to) test setting (field vs. lab), random sampling, designing and hypothesizing. We will also discuss procedures for quantitative data analysis and adequate forms of documentation. To deepen the knowledge, the lecture is accompanied by practical training courses that link theoretical findings to systems and applications in the field of human-computer interaction.