This article by HowStuffWorks.com explains at a chemistry / biology level how sunless tanning products works (including a diagram showing the layers of the skin (dermis and epidermis).

Article Excerpt: "... skin is made up of two 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..."

This article refers to the beginning of sunless tanning products, and addresses tanning on the outside (using bronzers and tanning lotions and sprays), tanning on the inside (using tanning pills and tanning accelerators) and answers the question 'Why Do Tans Fade?'.

Article Excerpt: "Self-tanning products have been around in some form or other since the invention of cosmetics..."