Some customers will find that a facial serum will give their skin enough moisture and they do not need an added moisturizer, while others will like using both. If you are using a serum, you apply it after cleansing your face and before applying moisturizer. If you want to use both a serum and moisturizer but do not want a heavy application and are concerned about using it under makeup, try using the serum at night. You might even alternate nights - using the serum one night and your moisturizer the next.
Here is a chart created by a fellow Avon Rep. showing the serums and other treatment products that Avon offers and their benefits. (Treatments are also used after cleansing and before moisturizing.) You can find all these products in my eStore.