Silage fermentation is an uncontrolled process affected by a multitude of forage factors and management practices. In general, undesirable fermentations result in excessive losses of DM and energy, and can result in an accumulation of unwanted end products that can negatively affect animal performance. Various types of additives including enzymes, organic acids, and microbial inoculants have been used to maintain and improve forage quality and improve aerobic stability during ensiling. For example, plant cell wall degrading enzymes have been added to silage to improve digestibility. However, true pre-digestion of nutrients in the silo is a questionable goal. Instead, the goal should be to improve the digestibility of silage when it reaches the rumen and or intestine of the cow. Short chain organic acids such as propionic acid, potassium sorbate, and sodium benzoate have good antifungal properties that can suppress yeasts that are responsible for initiating aerobic spoilage. Various microbial organisms have also been added to silages to improve fermentation. Originally classified as homolactic acid bacteria, such organisms were used to hasten silage fermentation and result in fermentations that recovered more DM and energy. Ironically, extremely efficient fermentations can lead to poor aerobic stability of silages. Over a decade ago, the introduction of L. buchneri, a heterolactic acid bacteria, resulted in a paradigm shift because it was able to anaerobically convert moderate amounts of lactic acid to acetic acid, which is highly antifungal in nature and thus improved aerobic stability. Changes in silage fermentation and aerobic stability from the use of additives can have direct and indirect effects on animal performance. This review will give a history of various silage additives, their modes of action and effects on animal production.