In this fast-advancing technology world, almost everything is written as software or application. Cybersecurity, or information security, has always been a very broad and comprehensive field and has been a fast-evolving area for the past 10-20 years. Within, there are many domains, such as risk management, security operations, network and infrastructure security, identity access management, and others. This book focuses on one particular domain called application security (AppSec). That's because, in today's modern world, software development has become the core of any product or service. As such, ensuring the security of any product or application development is critical to the success of your business. This book is a collection of wisdom from 77 security experts in application security across various industries. Organized into 12 topics, the book covers web applications, mobile applications, APIs, and the Internet of Things (IoT) (embedded systems). It also expands the safeguards to both on-prem and in-cloud development. More importantly, it explains all angles of AppSec such as secure software development life cycle (SDLC) practice, threat modeling, code scanning and testing, vulnerability management, and how to run a successful application security program. The book also provides insight into two emerging topics: software supply chain security and AI security. It is a treasure trove of those security practitioners' practical advice, distilled into bite-sized essays for both beginners and seasoned professionals in application security and cybersecurity.