Food fermentation is the oldest known food production and preservation method that has been used in vegetable and animal raw materials in different parts of the world since the Neolithic times. It is stated that there are more than 3500 fermented foods all over the world produced from fruits, vegetables, meat, grains, legumes and milk. During the fermentation process, the raw material changes and the product has different features. The main factors affecting the properties of fermented foods are the composition of the substrates in the raw material, the fermenting microorganisms and fermentation conditions, and the trace paths used by microorganisms. Various bacteria, yeast and molds play a role in fermentation. On the other hand, lactic acid bacteria are the most common microorganisms found in fermented products. During fermentation, many biochemical events take place, including changes in the composition of foods, and an increase in the properties of the product such as bioactivity and digestibility. It is known that fermentative products are more nutritious than the raw materials from which they are produced. In this article, we will try to give information about fermentation metabolism on the basis of substrates used in fermented food production, microorganisms, types of fermentation and changes in nutrients.