Glaucoma causes death of RGCs and their axons. Thus, its structural effects can be measured both by loss of thickness of the RGCs and RNFL and by topographical changes of the ONH. Glaucomatous functional deficits similarly result from RGCs loss with defects in visual sensitivity found in the receptive fields of the damaged neurons.The relationship between structure and function is fundamental to understand the disease and to differentiate damage due to glaucoma from other diseases of the ONH. There is evidence that RGC damage may occur in glaucoma before visual field defects are seen. However, several studies reported that the earliest damage in glaucoma patients can be either structural or functional in nature.In the present essay, different instruments which are used in the evaluation of structural damage in early glaucoma by RNFL analysis, ganglion cell layer analysis and optic nerve topography have been dicussed. Automated perimetry techniques which evaluate functional glaucomatous damage by visual field testing have been also discussed.Moreover, the debates concerning early diagnosis and progression of glaucoma by the evaluation of structural and functional damage have been highlighted. It could be concluded that the diagnosis of early glaucoma cannot be based on automated perimetry alone or OCT, HRT and SLP alone, but, the results of these tests should be interpreted together to increase diagnostic accuracy