The human gut microbiota functions as a critical component in maintaining metabolic balance, immune regulation, and intestinal homeostasis. Probiotics support host health by balancing the gut microbiota, preventing pathogen colonization, and modulating the immune system, while prebiotics enhance microbial diversity by selectively stimulating the growth of beneficial microorganisms. Synbiotics, which combine these two components, can provide additional benefits to gastrointestinal and systemic health through synergistic effects. The biological effects of probiotic and prebiotic supplementation have been investigated in numerous clinical and experimental studies across a wide spectrum, including gastrointestinal system disorders, atopic dermatitis in the pediatric period, immune-mediated allergic reactions, respiratory tract infections, obesity and related metabolic dysfunctions, cardiovascular diseases, neurocognitive processes, psychosocial health, and bone metabolism. It has been reported that these biotics modulate communication between the gut and other organs through microbial metabolites such as short-chain fatty acids and exert significant effects on systemic health. In this chapter, the mechanisms of action of probiotics, prebiotics, and synbiotics, along with their therapeutic and protective effects on human health, are addressed.