The understanding of masticatory muscle function and its relationship with craniofacial morphology is essential to the field of orthodontics. Electromyography (EMG) has been used to assess muscle function both qualitatively and quantitatively. Many studies attempted to relate masticatory muscle activity with facial form, but the results have been inconsistent. The influence of the masticatory muscles on orthodontic treatment, specifically vertical malocclusion correction, and stability is still controversial. The purpose of this article is to review the relationship among masticatory muscle function, facial morphology and malocclusion based on the electromyographic studies.