Although the correct filling of leadership positions is one of the most important investment decisions of any company, suboptimal methods are often used at this critical point. The most glaring consequence of this deficiency is the phenomenon of derailment: newly appointed leaders fail after a few months in their function and must leave the company again or are at least demoted. Derailment is only the tip of the iceberg of personnel misdecisions. The aim of this chapter is to introduce HR managers to the principles of a diagnostically professional personnel selection of leaders. For this purpose, all components of a meaningful approach from the analysis of the requirements to the assessment center are presented successively and consistently explained using a real practical example—the filling of a management position.