In our day-to-day life, we use different symbols, numbers, characters, letters, and so on. Where a numbering system is concerned, we are familiar with the decimal number system, which uses ten different digits (0, 1, 2, …, 9) to represent a number. But, a computer system only understands two digits: either 0 or 1. When we make a number with these two digits (0 or 1), we call it a binary number. Number systems like octal and hexadecimal also have a significant role in reducing binary representation size. This chapter explains everything about these number systems, such as the different symbols used to represent a number, their base, how to represent these numbers, and conversion among number systems.