People were always difficult to except the fact that something is different from themselves or their way of thinking. It is probably one of the reasons why numerical systems other than decimal are hard to understand. Still, whether we like it or not, reality is quite different. Decimal system used in everyday life is by far less used than binary code, which is the working basis for millions of computers across the world. Each numerical system rests upon its basis. With decimal numerical system, this basis is 10, with binary it is 2, while with hexadecimal it equals 16. Value of each digit depends on it’s position in the number, represented in certain numerical system. Sum of values of each digit is the value of the number. Binary and hexadecimal systems are especially interesting for this course. Besides these two, decimal system will be detailed, too, for the sake of comparison. Although there is nothing new to tell about decimal system, we will give it a look for its relations with other numerical systems.