This depends on the ingredients in your anti-aging serum and on your skin type. Serums that contain retinol are indicated for use in the evening as they may cause sensitivity to the sun when used in the morning. Anti-aging serums containing antioxidants, like vitamin c, should be used in the morning. Hyaluronic acid serums can be used in the morning and/or evening depending on your routine. For those with dry, sensitive skin, it is important to start by using a serum 2-3 times a week until you’ve gradually built up a tolerance for everyday use.