Vesicles are formed on the endothelial cells due to the acidification of the ocular environment and subsequent hypoxia when the eye receives a reduced amount of oxygen. Endothelial vesicles may occur in almost every contact lens wearer. They are detectable during slit-lamp examinations. However, endothelium cells always return to their original state when the problem is gone. The most common way to avoid these vesicles is to select contact lenses with high oxygen permeability, which minimize the process of acidification in the ocular environment.