The Peer Assessment Rating (PAR) index is a valid and reliable measure of orthodontic treatment outcome and is the most widely accepted such index. Assessing outcomes with the PAR index requires the examination of pre-treatment and post-treatment orthodontic study models. Beginning with the pre-treatment models, a score is given to each feature that deviates from an ideal occlusion (all anatomical contact points adjacent, good interdigitation between posterior teeth and non-excessive overjet/overbite), the scores are then added together to give a total that represents the severity of the malocclusion. The process is then repeated with the post-treatment models. The difference between the pre-treatment and the post-treatment scores reflects the improvement that has taken place during treatment. A score of zero represents an ideal occlusion and in general the higher the score, the more extensive the malocclusion. It is currently a condition of the NHS orthodontic contract for providers to monitor a proportion of their cases using the PAR index. This paper aims to provide primary dental care practitioners with an overview of the PAR index and should provide a useful guide for those wishing to seek calibration in the use of the index.