Aquaculture has become the fastest growing sector of food production in the world. Despite the encouraging trends, several constraints have a negative impact on the growth of aquaculture. Among those, diseases are the primary limiting factors. Bacterial diseases are responsible for heavy mortality in both wild and cultured fish. Antibiotics are used to control such infection but may result in development and spread of antimicrobial-resistant bacteria and resistance genes and occurrence of antimicrobial residues in fish tissues. This may induce a negative impact on human health, fish performance and the environment. An alternative approach employed recently to control bacterial infection in fish is the use of vaccines. However, vaccines are not available in many countries, a tremendous amount of human labour is required to deliver the vaccines, and implementation is also associated with handling stress for the fish, which may impair the efficacy of the vaccines to a great extent. The concept of biological disease control, particularly using nonpathogenic biological agents (probiotics), has received widespread attention during the last decade. This review focused on the nature, sources, benefit, selection criteria, mode of action and efficiency of probiotics.