Probiotic bacteria are being explored as biological control agents in aquaculture to reduce the reliance on antibiotics and improve fish and shellfish health. The review discusses the definition of probiotics, their potential modes of action, and the rationale for their use in aquaculture. Probiotics are defined as live microbial agents that beneficially affect the host by modifying the microbial community, improving feed utilization, enhancing immune response, or improving water quality. They are used in aquaculture to prevent disease, improve survival rates, and reduce the use of antimicrobial drugs. The review highlights that probiotics can be used in various stages of aquaculture, including fish eggs and larvae, fish juveniles and adults, crustaceans, and bivalve mollusks. They are applied in feed, culture tanks, or as part of live food. Probiotics can act through several mechanisms, including the production of inhibitory compounds, competition for nutrients or energy, competition for adhesion sites, and enhancement of the immune response. The review also discusses the challenges in selecting and developing probiotics, including the need for extensive testing and the importance of considering the environment in which they are used. The review emphasizes that while probiotics show promise in reducing disease and improving aquaculture productivity, their effectiveness can be influenced by factors such as the environment, the host's immune system, and the specific probiotic strain used. The review concludes that further research is needed to fully understand the mechanisms of action of probiotics and to develop effective strategies for their use in aquaculture.Probiotic bacteria are being explored as biological control agents in aquaculture to reduce the reliance on antibiotics and improve fish and shellfish health. The review discusses the definition of probiotics, their potential modes of action, and the rationale for their use in aquaculture. Probiotics are defined as live microbial agents that beneficially affect the host by modifying the microbial community, improving feed utilization, enhancing immune response, or improving water quality. They are used in aquaculture to prevent disease, improve survival rates, and reduce the use of antimicrobial drugs. The review highlights that probiotics can be used in various stages of aquaculture, including fish eggs and larvae, fish juveniles and adults, crustaceans, and bivalve mollusks. They are applied in feed, culture tanks, or as part of live food. Probiotics can act through several mechanisms, including the production of inhibitory compounds, competition for nutrients or energy, competition for adhesion sites, and enhancement of the immune response. The review also discusses the challenges in selecting and developing probiotics, including the need for extensive testing and the importance of considering the environment in which they are used. The review emphasizes that while probiotics show promise in reducing disease and improving aquaculture productivity, their effectiveness can be influenced by factors such as the environment, the host's immune system, and the specific probiotic strain used. The review concludes that further research is needed to fully understand the mechanisms of action of probiotics and to develop effective strategies for their use in aquaculture.