Tattooing involves the placement of pigment into the skin's dermis, the layer of dermal tissue underlying the epidermis. After initial injection, pigment is dispersed throughout a homogenized damaged layer down through the epidermis and upper dermis, in both of which the presence of foreign material activates the immune system's phagocytes to engulf the pigment particles. As healing proceeds, the damaged epidermis flakes away (eliminating surface pigment) while deeper in the skin granulation tissue forms, which is later converted to connective tissue by collagen growth. This mends the upper dermis, where pigment remains trapped within successive generations of macrophages, ultimately concentrating in a layer just below the dermis/epidermis boundary. Its presence there is stable, but in the long term (decades) the pigment tends to migrate deeper into the dermis, accounting for the degraded detail of old tattoos.
Mainstream art galleries hold exhibitions of both conventional and custom tattoo designs, such as Beyond Skin, at the Museum of Croydon. Copyrighted tattoo designs that are mass-produced and sent to tattoo artists are known as "flash", a notable instance of industrial design. Flash sheets are prominently displayed in many tattoo parlors for the purpose of providing both inspiration and ready-made tattoo images to customers.