This book discusses in detail the professional expertise of a school unit director. Expertise is nothing but a set of knowledge, skills, abilities, attributes, values, and attitudes that contribute to solving the professional practice problems of the school leader. The professional's expertise consists of a body of professional knowledge, general and technical skills, as well as performance abilities on the job. However, the demands of the modern era impose competence as the ultimate professional virtue, which is not the privilege of the elite, like knowledge, but of those who can demonstrate excellent performance in the execution of work, which, in turn, leads to increased productivity. The competence movement and the emphasis on leadership that started from the Anglo-Saxon world have now begun to spread through the influence of International Organizations in many countries around the world, even in those that previously had centralized education systems. This study, therefore, attempts to describe and analyze all those concepts that constitute part of the professional expertise for every prospective and active school unit director. It also presents the reality that applies to school unit directors in various countries around the world in order to contrast it with the corresponding Greek one, so as to highlight the lag that the latter seems to have in terms of international developments.