This chapter provides an overview of probiotics and prebiotics in human health. It reveals that there is a growing understanding of the role played by the resident microbiota on human health, including the prevention or amelioration of a number of conditions and symptoms. Probiotics can be defined as organisms and substances that contribute to the intestinal microbial balance. It is a live microbial feed supplement that beneficially affects the host animal by improving its intestinal microbial balance. A prebiotic is a nonviable food component that confers a health benefit on the host, associated with modulation of the microbiota. While probiotics are foreign microbial agents that act as enzymatic systems and stimulate the local and systemic immune system, prebiotics act by stimulating colonic fermentative processes and enhancing the proliferation of endogenous species and strains of bacteria. Prebiotics exert additional systemic effects through the stimulation of endocrine mechanisms that regulate satiety and weight gain. © 2010 Elsevier Inc. All rights reserved.