Nickel is a unique, mysterious and troublesome chemical element. Its molecular structure (unfilled electron shell) determines the high-reactivity and multidirectional biological effects. Some authors classify nickel as trace element, although its biological role in animal and human metabolism remains unclear. Conversely, nickel possesses strong sensitizing potential: as many as 65 million Europeans may be allergic to nickel. In this article, we review chemical and biological properties of nickel, pathomechanism, clinical symptoms and diagnosis of contact allergy to nickel, epidemiology and risk factors. Finally, public health measures and legal regulations of the EU aimed at protecting the population from nickel allergy are discussed, with particular attention devoted to the 'Nickel Directive' 94/27/EC.