This book has been developed to provide a detailed discussion of probiotics, which have been evaluated for use predominantly in fish and shellfish aquaculture. This book highlights strengths and weaknesses in knowledge and discusses gaps that need to be addressed. There has been a great deal of research concerning the use of probiotics in aquaculture. To date, a wide range of Gram-positive and Gram-negative bacteria and some eukaryotes, i.e. yeasts and unicellular algae, has been reported to be beneficial to aquatic hosts.  Following oral uptake, benefit includes improved growth performance and protection against many bacteria and some parasitic diseases. This book will be suitable for scientists, veterinarians, professionals, and senior students involved in aquaculture.