The practice of vocational education is very different in the European countries. We know systems which are entirely based on learning professional skills and acquiring professional knowledge at school. We know other systems which are completely based on in-company- training. The German system of vocational education, however, combines the two didactical approaches in a dual system of vocational education. Whereas inside of Germany there are heavy debates on its efficiency, its success is still widely recognised in other countries. The question which arouse from this observation was, how this success could be explained. The idea that simply both ways of acquiring professionality be necessary does not seem substantial enough to give appropriate explanation: If there was a way of building a didactically adequate link between theory at school and practice in the company, it should be possible to enable some sort of transformation from the one form of vocational learning to the other. A key to this problem is given by the Copenhague explanation of "comlementarity" by Nobel-prize winner Niels BOHR. Though he was concerned with the explanation of the phenomenon of "light" which can be proved to be a particle just as much as it can be proved to be a wave in other experiments. In physics, "particle" and "wave"are contradictory. This made physicists rather helpless. BOHR came up to declare "particle" and "wave" "complementary" to the phenomenon of "light". This means they cannot be considered "particle" and "wave" at the same time, for they are contradictory. However, the researcher has to be aware that only both give a full description of the phenomenon taken into consideration. Morphologically, the relationship between "theory" and "practice" is the same. They are just as much two contradictory approaches to vocational learning: for sure they cannot be taught at the same time, in most cases neither they can in the same place. Only if both ways of learning and teaching come to their right though, substantial professional competence can be the result of the intended didactical process. This explains the great success of the dual system in vocational education, which had been implemented about a century ago on the foundations of craftsmens' apprenticeship which is going back to the medieval age all over central Europe. In monal systems the learning of holistic knowledge and behaviour - e.g. the role of social relation in professional life - has to be experienced by the young person after the end of school. This has caused serious insertion problems in labour markets of the respective countries. In a monal system of in-company-training, very often the appropriate guideline theory, indispensible to autonomous professional action, is substituted by every- day's knowledge. This observation gives way to a new theory of education. It acknowledges the goals of "individual autonomy" or "emancipation" to education and puts it on a new basis. It also contributes to learning theory by delivering a new hypothesis: Successful learning cannot solely be explained by a theory of action as in MILLER, GALANTER & PRIBRAM 1960. Nevertheless, it can neither be fully explained by an structural or procedural approach on cognition as in AUSUBEL 1968. "Cognition" and "action" become the complementary categories of a new learning theory: Contradictory to one another, they are necessary to explain substantial learning which does neither lead to inert knowledge nor to limited (reflexologic) learning. Lit.: AUSUBEL, D.P.: Educational Psychology. A Cognitive View. New York: Holt, Rinehart & Winston 1968; JONGEBLOED, H.-C. (ed.): Wirtschaftspädagogik als Wissenschaft und Praxis - oder: Auf dem Wege zur Komplementarität als Prinzip (Moderne der Tradition 1). Kiel: Bajosch 1998; MILLER, G.A., GALANTER, E. & PRIBRAM K.H.: Plans and the Structure of Behavior. New York: Holt 1960.