Mystery shopper research (or mystery buyer research) is an often underestimated instrument with respect to its various applications. In industries where personal selling is the most important or one of the most important marketing mix variables, MSR can provide the management with a practical tool for control-purposes. It can be used very adequately for evaluating marketing objectives and specific marketing action programmes. MSR for instance can show whether the employee of a travel agency uses its sales tools ( e.g. computerterminal) in the prescribed way and whether he/she follows under certain circumstances the procedures set by central management. Evaluating the performance of the employee c.q. the branch office in an adequate way by questioning customers is quite an impossible task, because it is very difficult to operationalise the standards set by the central management in a questionnaire. There are however some strict rules under which MSR has to take place. This paper will give recommendations for using the instrument MSR and shows some overall results ending with some remarks with respect to the ethics involved. Finally it presents some results of our research about the validity of the instrument and gives a quantitative view on interviewers-bias in MSR.