The crimes made by the under-ages – is one of the most important problem in the society. The number of those crimes are growing very fast during the last years. So, that’s why it is so important to understand the factors of this negative activity, to start crime prevention on it’s early stage. While studying the earliest stages of prevention of under-ages crimes it is very important to analyze all the factors which can make influence for the activity like that. That’s why in this master’s work I tried to educe these tasks : 1. To describe the conception of crimes of under-ages and to educe the situation of nowadays. 2. To find and analyze the reasons, why under-ages are making crimes. 3. The importance of interdepartmental cooperation while trying to prevent the under-ages crimes. All those tasks were implemented, while analyzing science literature. Investigation showed, that the prevention of under-ages crimes must be shared by the society and all responsible officers. It is important, because only in that way good results can be reached. The first step of the prevention must be made in each family. Parents have to ensure not only physiological environment, but also to organize interesting leisure time. In the cases, when parents are disabled to do all those duties it is social workers, who are always ready to help under-ages, to feel a real member of society.