If you know that skin is made up of three main layers: the epidermis on the outside and the dermis on the inside. Whether you are talking about sun tanning or self-tanning, the epidermis is where the action occurs. The epidermis is also made up of layers. The deepest layer of the epidermis, called the stratum basale (basal layer), is affected during sun tanning. The stratum corneum (horny layer) is the outermost layer of the epidermis -- it is this layer that is affected by most sunless-tanning products.