In the world of digital technology, from the simplest calculator to the most complex supercomputer, information is processed and stored not as analog signals but as discrete values. This fundamental reliance on distinct states necessitates a clear understanding of number systems—the various ways we represent quantities. While humans commonly use the decimal (base-10) system, digital systems predominantly operate using binary (base-2), a system built on just two symbols: 0 and 1. Beyond simply representing numbers, codes provide the means to encode diverse types of information, such as characters, instructions, and error-checking data, into these digital formats. This chapter will lay the groundwork for understanding how these essential systems function, their interrelationships, and their critical role in the design and operation of all digital devices.