Mouth breathing, according to Angle, "is the most powerful cause, constant and varied in its results, causing the so-called malocclusions and an asymmetrical development of the facial muscles linked to the process itself"; its early diagnosis and individualisation of the cause are essential for a more favourable prognosis for the patient. The aim of this book is to provide the dental professional with further clarification, in the context of the origin being able to be situated in a daily routine environment, in order to seek out the most suitable options for the patient in solving this problem.