Periodontal bony defects are classified into ‘supraosseous’ (‘suprabony’) or ‘infraosseous’ (‘infrabony’) according to the location of the base of the defect compared to the coronal part of the residual alveolar crest. Infraosseous defects are generally considered more challenging to treat and are thought to be associated with a higher risk of periodontal progression. The emergence and advancement of periodontal regenerative procedures have improved the clinician’s ability to manage infraosseous defects. However, limitations still exist. This paper reviews the definitions of periodontal osseous defects and provides a new classification framework for infraosseous defects, relating them to the chances of success of regenerative procedures and therefore to their treatment planning options. Infraosseous defects are hereby divided into intrabony and inter-root defects. Factors affecting treatment response, such as number of walls, depth and extension into buccal and lingual surfaces are added to the classification framework.