This book provides a comprehensive and clinically oriented overview of rhinosinusitis. It covers fundamental concepts, pathophysiology, diagnostic approaches, and contemporary management strategies, including medical therapy, functional endoscopic sinus surgery, and minimally invasive techniques. Special emphasis is placed on fungal sinusitis, olfactory disorders, sinus headache, fever of unknown origin, imaging, and emerging challenges such as COVID-19–related sinonasal disease. Written in a clear and systematic manner, this book aims to serve as a practical reference for undergraduate and postgraduate students, as well as a useful guide for practicing otorhinolaryngologists.