Tattoos cause a broad range of clinical problems. Mild complaints, especially sensitivity to sun, are very common and seen in 1/5 of cases. Medical complications are dominated by allergy to tattoo pigment haptens or haptens generated in the skin, especially in red tattoos but also in blue and green tattoos. Symptoms are major and can be compared to cumbersome pruritic skin diseases. Tattoo allergies and local reactions show distinct clinical manifestations, with plaque-like, excessive hyperkeratotic, ulcero-necrotic, lymphopathic, neuro-sensory, and scar patterns. Reactions in black tattoos are papulo-nodular and non-allergic and associated with the agglomeration of nanoparticulate carbon black. Tattoo complications include effects on general health conditions and complications in the psycho-social sphere. Tattoo infections with bacteria, especially staphylococci, which may be resistant to multiple antibiotics, may be prominent and may progress into life-threatening sepsis. Contaminated tattoo ink is an open-window risk vector that can lead to epidemic tattoo infections across national borders due to contaminated bulk production. Hepatitis B and C and human immunodeficiency virus (HIV) transferred by tattooing remain a significant risk needing active prevention. It is noteworthy that cancer arising in tattoos, in regional lymph nodes, and in other organs due to tattoo pigments and ingredients has not been detected or noted as a significant clinical problem hitherto, despite millions of people being tattooed for decennia. Clinical observation and epidemiology disagree with register data, which indicate an increased risk of cancer due to chemical carcinogens present in some inks. Registers rely on chronic dosaging of cell lines and animals. However, tattooing in humans is essentially a single-dose exposure, which might explain the observed discrepancy.