Applied during the treatment step of a routine, the order of application is Essence then Serum or Ampoule. Essence is the lightest in consistency and can be absorbed faster. Think of it as a hybrid of a toner and a serum. It preps the skin for the next products just like toners but encourages cellular turnover. Serums are concentrated to target specific concerns like hyperpigmentation, hydration, damage repair, etc. Ampoules are serums but with higher concentration and many active ingredients.