We meet the applications of the results of number theory daily, maybe unwittingly. A variety of systems of identification numbers, such as the postal slips (USPS-The United States Postal Service), in the barcodes (UPC-Universal Product Codes) or books (ISBN-International Standard Book Number). Furthermore, the results of the theory of numbers used for generating random numbers. You shall also apply them in various areas. In addition to statistics find its place even in the theoretical physics-particle simulations. Probably the most important applications has number theory in cryptography, are based on it the extremly safe encryption methods, yet easily applicable in practice. In the subject of elementary number theory students should acquire basic knowledge of mathematical apparatus, which stands for the above applications. Then they can understand how these applications work in practice.