Bone defects are common and are associated with a significant burden of disease. The treatment of these injuries remains controversial, particularly those defects which are critical sized. Despite the need for decision making to be evidence based, a lack of consensus around definitions of critical-sized defects still exists, particularly around those defects in the 1-3 cm range. There is a need to define "critical" in bone defect size because noncritical defects may heal without planned reconstruction and secondary surgery. This article reviews the current evidence around the definition of a critical-sized bone defect and concludes that defects in the order of 2.5 cm or greater seem to have a poor natural history.