A tattoo is an ink design inserted into the skin, commonly via a needle. The tattoo, in various forms, has been used ornamentally and religiously by humans for thousands of years, with examples of tattoo work found on numerous preserved prehistoric specimens. Humans also use identification tattoos on domesticated animals, particularly livestock. Examples of tattoo work can be seen in most human cultures, and despite some social stigma, tattoos are becoming ubiquitous in the West, with an estimated 25% of Americans wearing at least one tattoo by the end of the 20th century.

The word tattoo is likely related to the Samoan tatau, meaning 'to strike or mark'. Tattoos became popularized in the Western world when sailors began to explore the Pacific and return with tattoo work. In Japan, where there is a long historical tradition of tattoo, the word irezumi refers to traditional Japanese tattoos, while tattoo is used in discussions of other types of tattoo art. Tattoo owners sometimes shorten the word to tat or use the terms ink, art, or work to talk about the designs they wear.

People receive tattoos for a variety of reasons: to identify themselves with a religious or social group, to adorn their bodies, as protective symbols, to cover skin discolorations, or as ongoing art and social projects. Most tattoo artists are themselves heavily tattooed. Some individuals have been forcibly tattooed, most notably victims of the Holocaust and prisoners.

Prehistoric tattoos were likely created by scoring the flesh with knives and rubbing in ink, ash, or another dye agent. These tattoos were probably more susceptible to infection, and also less detailed than modern tattoos. Most of the extant examples consist of lines and dots on various points of the body. The introduction of needles made from bone and wood to the art of tattoo began hundreds of years ago and made for more precision, less infection, and less painful work. Many traditional tattoos are still hand poked with tools such as animal bone, sharpened bamboo, or steel.

However, Thomas Edison's invention of the autographic printer in 1876 paved the way for an electric tattoo machine capable of striking the skin hundreds of times in a minute, making tattoos faster and much more widespread. Modern tattoo machines are strikingly different in operation than Edison's invention, although the same basic principle is followed. An electric tattoo machine operates using an electromagnet, and as the circuit is opened and closed, it causes a bar connected to the needle to move. Depending on the speed setting, the needle can move between 80 and 120 times a second, allowing the artist to penetrate the skin without laborious hand work.

A variety of pigments and inks are used in modern tattoo, ranging from traditional black to a wide range of colors. Some of the colors used for tattoo pigment may be toxic, raising concerns about extensive color work. If concerned, ask the artist about what pigments he or she is using and whether any adverse reactions to the inks have been noticed. Many tattoos will also require touchup, as exposure to sunlight and water degrades the inks.

When receiving a tattoo, it is important to make sure that proper hygienic measures are taken. Make sure that the studio is clean and that the artist is wearing gloves and using autoclaved needles. Most tattoo artists keep their work areas scrupulously clean, only laying out the materials they need to perform your tattoo. Artists have varying aftercare instructions for a new tattoo, and it is generally advisable to follow the directions from the artist who performed your tattoo for quick, clean, beautiful healing.