I wouldn't exactly call the product a serum; it's more of a very watery moisturizer. Just as what Nivea said, the product sinks into the skin easily and quickly. It's almost like you didn't apply anything. During my first week of usage, I noticed that my skin was kept hydrated throughout the day even though I only applied it at night. In my second week, I realized that my skin looked brighter and clearer, especially on my arms. But I have been so busy that I didn't think much of it until I looked through my photos.