Cryptography is a vital technology that underpins the security of information in computer networks. This book presents a comprehensive introduction to the role that cryptography plays in supporting digital security for everyday technologies such as the internet, mobile phones, Wi-Fi networks, payment cards and cryptocurrencies. This book is intended to be introductory, self-contained and widely accessible. It is suitable for a first read on cryptography. Almost no prior knowledge of mathematics is required since the book deliberately avoids the details of the mathematical techniques underpinning cryptographic mechanisms. Instead, it concerns what a normal user or practitioner of cyber security needs to know about cryptography in order to understand the design and use of everyday cryptographic applications. This includes the implementation of cryptography and key management. By focusing on the fundamental principles of modern cryptography rather than the technical details of the latest technology, the main part of the book is relatively timeless. The application of these principles illustrated by considering a number of contemporary uses of cryptography. These include emerging themes, such as post-quantum cryptography and the increased demand for cryptographic tools supporting privacy. The book also considers the wider societal impact of use of cryptography, including ransomware and the challenge of balancing the conflicting needs of society and national security when using cryptography. A reader of this book will not only be able to understand the everyday use of cryptography, but also be able to interpret future developments in this fascinating and crucially important area of technology.