biological viewpoint, with no detectable increase in the risk of the development of cancerous cells. It was not until the late 1980s that Q-switched lasers became commercially practical with the first marketed laser coming from Derma-lase Limited, Glasgow. One of the first American published articles describing laser tattoo removal was authored by a group at Massachusetts General Hospital in 1990. Tattoos consist of thousands of particles of tattoo pigment suspended in the skin. While normal human growth and healing processes will remove small foreign particles from the skin, tattoo pigment particles are too big to be removed automatically. Laser treatment causes tattoo pigment particles to heat up and fragment into smaller pieces. These smaller pieces are then removed by normal body processes. Q-switched lasers produce bursts of infrared light at specific frequencies that target a particular spectrum of color in the tattoo ink. The laser passes through the upper layers of the skin to target a specific pigment in the lower layers. Laser tattoo removal is a successful application of the theory of selective photothermolysis (SPTL). However, unlike treatments for blood vessels or hair, the mechanism required to shatter tattoo particles uses the photomechanical effect. In this situation the energy is absorbed by the ink particles in a very short time, typically nanoseconds. The surface temperature of the ink particles can rise to thousands of degrees but this energy profile rapidly collapses into a shock wave. This shock wave then propagates throughout the local tissue (the dermis) causing brittle structures to fragment. Hence tissues are largely unaffected since they simply vibrate as the shock wave passes. For laser tattoo removal the selective destruction of tattoo pigments depends on four factors: The color of the light must penetrate sufficiently deep into the skin to reach the tattoo pigment. Pigments deeper in the skin are harder to remove than those near the surface. The color of the laser light must be more highly absorbed by the tattoo pigment than the surrounding skin. Different tattoo pigments therefore require different laser colors. For example, red light is highly absorbed by green tattoo pigments, while yellow tends not to absorb light. The time duration (pulse duration) of the laser energy must be very short, so that the tattoo pigment is heated to fragmentation temperature before its heat can dissipate to the surrounding skin. Otherwise, heating of the surrounding tissue can cause burns or scars. For