Whether it is a text message or a video, everything in the digital world is represented by numbers. When working in IT, a fundamental skill is understanding how these numbers are structured, managed, and transformed. In this chapter, we introduce the concept of number systems, which are the basis of all data representation in computing. Then we explore the binary and hexadecimal number systems, which computers use to encode and process data. Finally, we learn how to convert between these number systems and perform arithmetic with them, enabling you to interpret and work with low-level data representations. Having a strong understanding of number systems is essential to mastering computer languages, which will help you debug code, analyze memory, and design algorithms.