This chapter provides an overview of the cellular architecture and functional specialization of the human eye. It outlines the sizes, locations, and densities of key ocular cells, including photoreceptors, retinal pigment epithelium, bipolar, ganglion, horizontal, amacrine, Müller glia, corneal, lens, and conjunctival cells. It examines age-related changes, minimum critical cell densities for functional vision, and factors that influence cellular health and degeneration, such as disease, genetics, oxidative stress, and trauma. The regenerative capacity of different cell types is discussed, with emphasis on the limited renewal ability of retinal and lens cells versus the active regeneration of corneal and conjunctival epithelium. Additionally, this chapter explores the physical limits of visual perception, including photon sensitivity, wavelength detection, spatial and temporal resolution, contrast sensitivity, and dynamic range, highlighting how cellular structure underpins the eye’s remarkable but finite optical performance.