Food is traditionally preserved via fermentation, which is used to increase food safety, prolong shelf life and improve food nutritional value and sensory appeal. In many places, especially Asia, Africa and portions of Europe, fish-based ethnic fermented food dishes constitute an essential component of traditional food. These foods are made with indigenous fermentation methods that use the growth of microorganisms like moulds, yeasts and lactic acid bacteria, either naturally or under supervision. Furthermore, fermented fish are an important part of the economies of many countries and are a reservoir of interesting microorganisms. Most of Southeast Asia utilizes fermented fish in rice. Northeast India eat fresh, preserved and fermented fish from rivers, streams and lakes. Some fish is preserved or prepared using indigenous fermenting, drying and smoking techniques. Using indigenous knowledge, ethnic groups treat fish without chemicals. In Northeast India, fermented fish products include shidal, lonailish, ngari, tungtap and hentak. Fish that has undergone fermentation not only has a longer shelf life but also has better nutritional value. As antinutritional substances and harmful bacteria are reduced, the process boosts the bioavailability of key nutrients, such as vitamins, proteins, peptides and omega-3 fatty acids. Antioxidant, antibacterial and anti-inflammatory bioactive chemicals are also produced when proteins and lipids are broken down by enzymes. This chapter focuses on fermented fish and their potential health benefits, as well as nutritional value.