Complex microbial ecosystems are useful in the production of fermented products from almost all types of food raw materials, including cereals, legumes, vegetables and fruits, milk, meat, and seafoods. The products such as bread, vinegar, alcoholic beverages, sauerkraut, olives, fermented milk products, sausages, seafoods, etc., are different in structures, tastes, avors, rheological properties, keeping quality, and nutritional value when compared to the raw materials. Different compounds are generated in the fermented foods as products of the catabolic reactions of the carbohydrates, proteins, and lipids, and the majority of these reactions are enzyme catalyzed. A wide range of bioactive compounds like polysaccharide polymers, peptides, and low-molecular-weight vitamins, antioxidants, etc., are produced, mostly by bacteria and fungi. There is a need to identify the most potent microorganisms, determine the molecular structures of unexplored bioactive compounds, and optimize yields. The different chapters of this book mainly focus on the chemical and property changes caused by fermentation and prefermentation processes.