This master’s final thesis is purposive to analyze the doctrine of direct effect of Community law, which is enlarged in the practice of the European Court of Justice. There are discussed the stages of the formation of the current doctrine identifying the concrete premises, which have been determined the enlargement of the principle of direct effect on the various sorts of forms of Community law. The main attention goes to the question of the direct effect of directives verifying the correctness of the position of the European Court of Justice not to recognize the horizontal direct effect of directives.In other hand, it is strived to verify the whole doctrine of direct effect in order to check the ability to use even the principle of supremacy of the Community law discussing the question of the collision between national and Community law.