A participant is a person that takes part in a research study. Depending on test object and problem statement appropriate participants for the usability study are selected. Thereby, it is the goal to select potential users that would use the test object in real life. At the same time, it is crucial that participants are unbiased when participating in the research method. They should not be trained or well grounded, but behave as naturally as they would behave beyond the test situation. For all usability studies including real users it is essential to observe participants behaving as natural as possible in order to identify relevant usability problems.