The general rule for applying skincare products is to layer them from thin to thick, and to apply the products with the highest percentage of active ingredients first. Therefore, serum should be applied before moisturizer. Serums are viewed as the "treatment" step in your skincare routine, and they contain higher concentrations of active ingredients than traditional creams or moisturizers. They are designed to penetrate deeply into the skin, providing targeted treatment for specific concerns and delivering powerful results. Applying serum before your moisturizer ensures that your skin gets the maximum benefit from its active ingredients. Moisturizers, on the other hand, are formulated with oils and are heavier and more protective to your skin. They are the thickest product in your skincare routine and stay closest to the surface of your skin. They impart benefits and moisture from the top down while sealing serums into your skin. Therefore, it is best to layer lighter products like serums before heavier products like moisturizers.