Maxillary transverse deficiency is one of the critical manifestations of malocclusion. It is of vital importance to diagnose maxillary transverse deficiency at the early stage because maxillary development in the transverse dimension stops early. Currently, there are several approaches to diagnose maxillary transverse deficiency, including clinical examination, dental cast measurement, poster-anterior cephalogram, and cone-beam CT. This narrative review will review these diagnostic and predictive approaches of maxillary transverse deficiency.