Probiotics are a group of live microorganisms (bacteria and/or yeasts) that exert beneficial effects on humans and animals, principally by balancing the intestinal flora. Probiotic microorganisms are mostly of human and animal origin and are normal constituents of their gastrointestinal microflora. However, several probiotics have been isolated from fermented foods. Various curative properties such as immune modulation, reducing serum cholesterol, anti-mutagenic and anti-carcinogenic effects, alleviation of lactose intolerance symptoms, strengthening defense mechanisms, improvement of intestinal barrier function and improving nutritional value have been attributed to probiotics. Regarding these health benefits, probiotics are considered as substantial ingredients in formulation of healthful and functional food products. This book presents an overview of the probiotic concept and its various functionalities in food products. Several scientific publications have been published in the field of probiotics, but due to the lack of books devoted to in vitro functionality of probiotics, a discrete need was recognized for a book to gather the basic and recent publications in this context while following a simple and applied approach. The chapter contributors of this book are renowned and expert authors in the probiotic field from key institutions around the world. We hope the book will be a useful reference for researchers, product developers, health professionals, students and regulators.