Nickel is a common component in many orthodontic materials. An allergy to nickel is commonly seen in the population, more frequently in women. This allergy has increased with the more frequent use of nickel containing jewellery and intraoral piercings. As a result, this allergy can be expected to be more readily encountered in dental practice. Possible allergy to nickel should be a question in the initial patient health history questionnaire. The dental practitioner should be mindful of this allergy during the course of orthodontic treatment, and know how to diagnose a nickel allergy if it appears and subsequent action in treatment and referral if it is suspected. This paper provides a summary of nickel allergy, its epidemiology, diagnosis and recommendations and alternatives to treatment. A detailed description of two cases where it was discovered in orthodontic patients is also reported.