It is generally accepted that glaucoma occurs when intraocular pressure (IOP) is raised above atmospheric pressure beyond tolerable limits for the optic disc. However, the other, unseen side of the optic disc is not air but a set of pressure compartments dominated by the cerebrospinal fluid (CSF) within the subarachnoid space. This invisibility has made investigation difficult; however, in recent decades there has been increased interest in this corollary to IOP. We briefly review the anatomy of the optic nerve subarachnoid space and its pressure relationships to intracranial, retrolaminar, and orbital tissue pressures. The CSF pressure is equivalent to IOP in its influence on translaminar pressure gradient and optic disk surface movement. At low CSF pressure, its influence on retrolaminar tissue pressure is reduced tending to minimize an increase in translaminar pressure gradient. The available evidence suggests that orbital tissue pressure provides this moderating influence. CSF pressure affects axonal transport, which is known to be important in glaucoma etiology and retinal venous outflow and pressures. Recent attempts to develop noninvasive measurement of CSF pressure have increased our knowledge of retinal venous changes in glaucoma. Further work in this area is likely to greatly increase our understanding of glaucoma.