Because it requires breaking the skin barrier, tattooing may carry health risks, including infection and allergic reactions. Modern tattooists reduce such risks by following universal precautions, working with single-use items, and sterilizing their equipment after each use. Many jurisdictions require that tattooists have bloodborne pathogen training, such as is provided through the Red Cross and OSHA.

Infection from tattooing in clean and modern tattoo studios employing single-use needles is rare. In amateur tattoos, such as those applied in prisons, however, there is an elevated risk of infection. Infections that can theoretically be transmitted by the use of unsterilized tattoo equipment or contaminated ink include surface infections of the skin, herpes simplex virus, tetanus, staph, fungal infections, some forms of hepatitis, tuberculosis, and HIV. In the United States there have been no reported cases of HIV contracted via commercially-applied tattooing process.

Tattoo inks have been described as "remarkably nonreactive histologically".[20] However, cases of allergic reactions to tattoo inks, particularly certain colors, have been medically documented. Occasionally, when a blood vessel is punctured during the tattooing procedure a bruise/hematoma may appear.

Tattoo artists, and people with tattoos, vary widely in their preferred methods of caring for new tattoos. Some artists recommend keeping a new tattoo wrapped for the first twenty-four hours, while others suggest removing temporary bandaging after two hours or less. Many tattooists advise against allowing too much contact with hot tub or pool water, or soaking in a tub for the first two weeks. This is to prevent the tattoo ink from washing out or fading due to over-hydration and to avoid infection from exposure to bacteria and chlorine. In contrast, other artists suggest that a new tattoo be bathed in very hot water early and often.

General consensus for care advises against removing the scab that forms on a new tattoo, and avoiding exposing one's tattoo to the sun for extended periods; both of these can contribute to fading of the image. Furthermore, it is agreed that a new tattoo needs to be kept clean. Various products may be recommended for application to the skin, ranging from those intended for the treatment of cuts, burns and scrapes, to cocoa butter, salves, lanolin, A&D or Aquaphor. Oil based ointments are almost always recommended to be used in very thin layers due to their inability to evaporate and therefore over-hydrate the already perforated skin. In recent years, specific commercial products have been developed for tattoo aftercare. Although opinions about these products vary, there is near total agreement that either alone or in addition to some other product, soap and warm water work well to keep a tattoo clean and free from infection. Ultimately, the amount of ink that remains in the skin throughout the healing process determines, in large part, how robust the final tattoo will look. If a tattoo becomes infected (uncommon but possible if one neglects to properly clean their tattoo) or if the scab falls off too soon (e.g., if it absorbs too much water and sloughs off early or is picked or scraped off), then the ink will not be properly fixed in the skin and the final image will be negatively affected.

In the UK, most tattoo artists recommend Bepanthen as the main aftercare product for new tattoos.

The properly equipped tattoo studio will use biohazard containers for objects that have come into contact with blood or bodily fluids, sharps containers for old needles, and an autoclave for sterilizing tools.[22] Certain jurisdictions also require studios by law to have a sink in the work area supplied with both hot and cold water.

Proper hygiene requires a body modification artist to wash his or her hands before starting to prepare a client for the stencil, between clients, and at any other time where cross contamination can occur. The use of single use disposable gloves is also mandatory. In some states and countries it is illegal to tattoo a minor even with parental consent, and (except in the case of medical tattoos) it is usually not allowed to tattoo impaired persons, people with contraindicated skin conditions, those who are pregnant or nursing, those incapable of consent due to mental incapacity or those under the influence of alcohol or drugs.

Before the tattooing begins the client is asked to approve the final position of the applied stencil. After approval is given the artist will open new, sterile needle packages in front of the client, and always use new, sterile or sterile disposable instruments and supplies, and fresh ink for each session (loaded into disposable ink caps which are discarded after each client). Also, all areas which may be touched with contaminated gloves will be wrapped in clear plastic to prevent cross-contamination. Equipment that cannot be autoclaved (such as counter tops, machines, and furniture) will be wiped with an approved disinfectant.[23]

Membership in professional organizations, or certificates of appreciation/achievement, generally helps artists to be aware of the latest trends. However, many of the most notable tattooists do not belong to any association. While specific requirements to become a tattooist vary between jurisdictions, many mandate only formal training in bloodborne pathogens, and cross contamination. The local department of health regulates tattoo studios in many jurisdictions.

For example, according to the health departments in Oregon and Hawaii, tattoo artists in these states are required to take and pass a test ascertaining their knowledge of health and safety precautions, as well as the current state regulations. Performing a tattoo in Oregon state without a proper and current license or in an unlicensed facility is a felony offense.[24] Tattooing was legalized in New York City in 1997,[25] and in Massachusetts and Oklahoma between 2002 and 2006.

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 fibroblasts, 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.

Some tribal cultures traditionally created tattoos by cutting designs into the skin and rubbing the resulting wound with ink, ashes or other agents; some cultures continue this practice, which may be an adjunct to scarification. Some cultures create tattooed marks by hand-tapping the ink into the skin using sharpened sticks or animal bones (made like needles) with clay formed disks or, in modern times, needles. Traditional Japanese tattoos (Horimono) are still "hand-poked," that is, the ink is inserted beneath the skin using non-electrical, hand-made and hand held tools with needles of sharpened bamboo or steel. This method is known as tebori.

Traditional two coil tattoo machine

The most common method of tattooing in modern times is the electric tattoo machine, which inserts ink into the skin via a group of needles that are soldered onto a bar, which is attached to an oscillating unit. The unit rapidly and repeatedly drives the needles in and out of the skin, usually 80 to 150 times a second. This modern procedure is ordinarily sanitary. The needles are single-use needles that come packaged individually. The tattoo artist must wash not only his or her hands, but they must also wash the area that will be tattooed. Gloves must be worn at all times and the wound must be wiped frequently with a wet disposable towel of some kind.

Prices for this service vary widely globally and locally, depending on the complexity of the tattoo, the skill and expertise of the artist, the attitude of the customer, the costs of running a business, the economics of supply and demand, etc. The time it takes to get a tattoo is in proportion with its size and complexity. A small one of simple design might take fifteen minutes, whereas an elaborate sleeve tattoo or back piece requires multiple sessions of several hours each.

The modern electric tattoo machine is far removed from the machine invented by Samuel O'Reilly in 1891. O'Reilly's machine was based on the rotary technology of the electric engraving device invented by Thomas Edison. Modern tattoo machines use electromagnetic coils. The first coil machine was patented by Thomas Riley in London, 1891 using a single coil. The first twin coil machine, the predecessor of the modern configuration, was invented by another Englishman, Alfred Charles South of London, in 1899.

A tattoo is a marking made by inserting indelible ink into the layers of skin to change the pigment for decorative or other reasons. Tattoos on humans are a type of decorative body modification, while tattoos on animals are most commonly used for identification or branding.

Tattooing has been practiced worldwide. The Ainu, the indigenous people of Japan, traditionally wore facial tattoos. Today one can find Berbers of Tamazgha (North Africa), Māori of New Zealand, Arabic people in East-Turkey and Atayal of Taiwan with facial tattoos. Tattooing was widespread among Polynesian peoples and among certain tribal groups in the Taiwan, Philippines, Borneo, Mentawai Islands, Africa, North America, South America, Mesoamerica, Europe, Japan, Cambodia, New Zealand and Micronesia. Despite some taboos surrounding tattooing, the art continues to be popular in many parts of the world.