Nowadays, companies are facing tremendous challenges, due to highly competitive pressure and increasing customer requirements. In order to gain higher customer retention and loyalty, B2B companies are using the strategic marketing or distribution instrument “Key Account Management”. Key accounts receive a special treatment by companies. Of course, additional costs occur. Now the question is, how a company can prove that the implementation of Key Account Management is profitable. The English term for this process is “Performance Measurement”. Thus, a suitable concept for the usage in Key Account Management is essential. First of all, this book contains a general description of Key Account Management in order to spot the need for the use of Performance Measurement. Of course, also Performance Measurement is specified and a special focus is put on various concepts fitting to Key Account Management. Furthermore, emphasis was placed on the difference of Performance Measurement in Global Account Management. Finally, the proposed performance measurement instrument and the most appropriate key performance indicators for the application in Key Account Management are pointed out.