This book provides a comprehensive examination of the role of Bacillus bacteria in aquaculture, particularly focusing on finfish and shellfish. It begins with foundational chapters on the anatomy and physiology of these aquatic species, their immune systems, and the biology of Bacillus bacteria. The book discusses the specific interactions between Bacillus and the gastrointestinal tracts of fish and shellfish, examining the production and effects of Bacillus-derived exoenzymes and bacteriocins and their role in enhancing aquatic animal health and water quality. Additionally, the book covers the role of Bacillus in bioremediation, particularly in improving water quality, which is a significant concern in aquaculture. The latter sections delve into the application of Bacillus as probiotics and paraprobiotics in fish and shellfish culture, their integration in feed biotechnology, and a critical evaluation of their safety. The book concludes with a critical discussion on the safety and regulatory aspects of using Bacillus in aquaculture. This book is intended for professionals and practitioners in the aquaculture industry and researchers, academicians, and students in the fields of microbiology, marine biology, aquaculture, and veterinary medicine.