Welcome to "Illuminating the Dark: A Comprehensive Guide to Photodetectors." This book delves into the intricate workings of photodetectors, devices pivotal in converting light into electrical signals across various fields. From the foundational principles of photon absorption and semiconductor physics to the diverse array of photodetector types such as photodiodes, phototransistors, and emerging technologies like quantum dot and organic photodetectors, each chapter explores their applications in telecommunications, imaging, and beyond. Practical insights on selecting, designing circuits, and optimizing performance complement case studies illustrating real-world implementations. As photodetectors evolve alongside technological advancements in AI and IoT integration, this book aims to provide a comprehensive understanding for students, engineers, and enthusiasts interested in the transformative role of light detection technologies.