Endoscopic third ventriculostomy (ETV) is one of the two surgical procedures for the treatment of hydrocephalus, its main indication being obstructive hydrocephalus. Its efficacy is related to the age of the patient and the etiology of the hydrocephalus; however, more studies appear where ETV has gained ground beyond obstructive hydrocephalus, and despite the fact that there is still a lack of evidence to issue a grade of recommendation. ETV has shown to be useful even in communicating hydrocephalus and in patients younger than 6 months. This chapter shows a summary of the most important points to take into account in this procedure. Likewise, the third endoscopic ventriculostomy gives us the opportunity to continue studying the intraventricular dynamics of the cerebrospinal fluid, the ventricular anatomy, the pathology around or within the ventricular system and other details that can open doors for us to understand the concept of hydrocephalus, improve its treatment and improve known surgical techniques.