When it comes to the order of applying serum and moisturizer, the general rule is to layer products 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 lightweight, highly concentrated formulas that penetrate deeply into the skin to deliver active ingredients, such as antioxidants, vitamins, and peptides. They are viewed as the "treatment" step in a skincare routine and are often the most potent product. On the other hand, moisturizers are used to hydrate and protect the skin from environmental damage. They work by creating a barrier on the skins surface to prevent moisture loss and keep the skin soft and supple.

However, there is one exception to this rule. If a serum contains potentially irritating ingredients like retinol or salicylic acid, applying a moisturizer first could help make the serum less potent and less irritating.

It is important to note that not everyone needs to use both serum and moisturizer, and the number of steps in a skincare routine can vary depending on individual skin type and concerns. When choosing a serum or moisturizer, it is important to research the products active ingredients and to avoid using too many active ingredients at once to avoid interactions and side effects.