The cone photoreceptor-specific cyclic nucleotide-gated (CNG) channel is indispensable for cone function. Cones are essential for daylight vision and visual acuity. Mutations in the CNGA3 and CNGB3 genes are associated with achromatopsia, cone degeneration, and early-onset macular degeneration, and account for 80-85% of Achromatopsia cases. Patients with CNG channel defects exhibit cone dysfunction and progressive degeneration of cones, as revealed by electrophysiological recordings, psychophysical testing, and morphological examinations. The cellular events and underlying mechanisms of CNG channel deficiency have been explored using mouse models. In this review, we have summarized our current understanding of the modes of cone defects due to CNG channel deficiency.