It is now established that orthodontic treatment increases the risk of carious lesions, with consequent harm to the patient and a high risk of compromising treatment outcome. This risk is related to the appliances, which increase the number of sites where plaque can accumulate as well as to changes in the bacterial flora and the age of the patient. A thorough evaluation of the risk of caries is therefore needed before any appliance is put in place and further evaluations should then be performed regularly throughout the course of treatment. In addition, preventive measures should be taken, involving first and foremost educating and motivating patients regarding the need for good oral hygiene. Prevention also implies adequate usage of the various sources of local fluoride administration, notably the application of fluoride-rich varnish. Other prophylactic measures such as using chlorhexidine varnish and sealing dental grooves are also recommended. Finally, the orthodontist can reduce to a minimum the use of items likely to retain dental plaque such as bands and elastomeric ties. The aim of this article is to suggest a specific diagnostic approach and to present the scientifically validated prevention measures, which need to be applied throughout orthodontic treatment.