Information theory is a mathematical theory of telecommunication. In these days we witness a wide-scale spreading of different data compressing and error-limiting methods in data transmission, so it is reasonable to survey their basic principles. The first chapter deals with
data compression without loss, the second one with source coding allowing distortion. The topic of the third chapter is channel coding. The fourth chapter comprises the bases of error fixing codes, the fifth one deals with open key cryptography. The latter field plays an ever increasing role in the era of the Internet.