In vertebrate eyes, vision begins when the photoreceptor's outer segment absorbs photons and generates a signal destined for the brain. As the locus of the origin of vision, the outer segment's structure and function are of great interest, to expand our understanding of both normal vision and visual dysfunction. The past fifteen years have seen astounding growth in our ability to observe this fundamental component of the living human visual system, largely due to the application of adaptive optics (AO) in the field of retinal imaging. AO has allowed unprecedented resolution of retinal structures; cones in particular have enjoyed hundreds of studies, in vivo, of their structure, size, topography, alignment, optical density, spectral properties, and sampling of the ocular image. Cone function, by contrast, has received comparatively little attention, for a few key reasons: the optical correlates of cone function are small and difficult to discriminate from the noise intrinsic to retinal imaging systems; physiological processes may be rapid, requiring correspondingly fast imaging systems; and imaging cellular function requires tracking of eye movements with sub-cellular precision. The main objective of this thesis is to use AO to observe and measure functional changes in living human cone photoreceptors. This effort requires the combined application of AO and other tools: 1) imaging systems capable of acquiring images of the retina with speed and signal sufficient to measure functional changes; 2) image processing software capable of tracking cones over thousands of images and extracting meaningful information about optical changes. These tools are used to investigate two aspects of cone function: phototransduction—the cascade of biochemical and structural changes evoked by visible stimulation of the cell—and cellular renewal—the process by which the cone maintains its structural and biochemical integrity.