Introduction: Excessive mouth breathing may be related to different oral and medical conditions. Objective: To explore the possible mechanisms involved in the relationship between mouth breathing and some oral and medical conditions. Methods:A review of the literature was performed by using PubMed, Scielo and Hinari. Relevant and updated articles were selected. Results: Mouth breathing has been linked to oral conditions such as dry mouth and lips, dental caries, periodontal  disease, secondary halitosis,craniofacialdeformity, malocclusion,abnormal swallowing, and to medical conditionssuch as obstructive sleep apnea, asthma,compromised airway, altered body posture, heart diseases and poor performance. Mechanisms that may  explain that relationship vary from biochemical, physiological and immunological deficiencies (chronic hypoxemia with  hypercapnia, increased water and energy lose, decreased growth hormone release, inflammatory and oxidative mediators release),to  anatomical effects (large load on the upper back and neck muscles,deformity in the airway passage, craniofacial deformities).Conclusions: Although the relationship between mouth breathing and oral and medical conditions seems well established, it is difficult to assessin all cases from the literature data,the cause-effect link.More studies are needed to explore a causal relationship.