Fats and oils are more than 'just' energy sources for animal feeds. The fatty acids as part of triglycerides or in Fatty Acid products differ in chemical composition and physical characteristics. The omega-3 and specifically the LC-PUFA omega-3 fatty acids can be considered as functional nutrients. Via the feeding of different fats and differing fat additions body composition and animal products can be modified. The main use of fats and oils in the feed industry is however increasing the energy content of the feed economically. Therefore knowledge about the digestibility of the fat for each animal category, the metabolisible energy content and the efficiency with which this energy can be used for different production goals are of paramount importance for evaluating the fat source of choice. This book has summarized the presently available knowledge from the scientific literature and concluded with a model to estimate the energy content in practice.