Hypotony is a natural occurrence, symptom, and complication of surgical treatment. With more sophisticated and aggressive techniques, postsurgical hypotony recently has been given increased attention as an obstacle to success of surgery for glaucoma and retinal detachment. Whereas two standard deviations below normal pressure (15.9-5.8 = 10.1 mm Hg) can be called hypotonous, most eyes, depending on scleral rigidity, lid pressure, eye rubbing, or corneal or retinal edema, will be symptomatic at < 5 mm Hg. Hypotony can be defined as the low pressure (whether acute, transient, chronic or permanent) which, in an individual eye, leads to functional changes (whether asymptomatic or symptomatic) and structural changes (whether reversible or irreversible). Depending on its duration and degree, postsurgical hypotony produces characteristic tissue changes that often are modified by, but separate from, the tissue changes caused by an underlying disease or its surgical treatment. This review summarizes the situations, variably associated with hypotony, that occur after such interventions as cataract extraction, filtering surgery, cyclodialysis, cyclodestruction, and vitreoretinal surgery, in addition to the reported pathomechanisms of hypotony and its proposed treatments.