The book "Trusted Execution Environments" offers a comprehensive analysis of Trusted Execution Environments (TEEs), which play a crucial role in protecting sensitive data and code across various computer platforms. TEEs are particularly important as they provide security even when the primary operating system is compromised. In recent years, TEEs have gained significance and are now deployed on billions of devices to safeguard financial transactions, personal data, and copyrighted content. Despite their widespread application, TEEs are often complex and poorly understood. This technical book fills this knowledge gap by detailing various TEE technologies, their features, advantages, and challenges. It provides a holistic view of secure and trusted executions, including an examination of smart cards and CPU protection rings, as well as modern TEEs like Intel SGX and ARM TrustZone. Additionally, relevant industry standards and specifications are discussed, and case studies are presented that illustrate the application of TEEs in common security mechanisms.