Ion channels are fundamental to the functioning of life, regulating processes as diverse as neural signaling, homeostasis, and environmental sensing, across the complexities of life from bacteria to the most advanced organisms. Among this vast diversity of ion channels, cyclic-nucleotide gated (CNG) channels hold particular significance and play a pivotal role in the sensory transduction across a variety of species. They transduce chemical signals into electrical signals, linking the external environment and our sensory perceptions. CNG channels were discovered almost 40 years ago and much knowledge has been gained on their physiological roles, biophysical properties, molecular characteristics, and channelopathies. However, the structural details of these channels remained elusive for a long time, mainly due to the lack of a full-length channel structure. It was only recently that atomic-resolution structures of full-length CNG channels became available, and structures of native mammalian CNG channels were only determined within the last two years. In my thesis, I use single particle cryogenic electron microscopy (cryo-EM) to determine the structures of native human cone CNGA3/CNGB3 channel in different biochemical environments and in different states, spanning the full spectrum of channel activation by its natural ligand cGMP. In addition, I use cryo-EM, electrophysiology, calcium imaging, and other biochemical techniques to characterize both wild-type and disease-associated mutant (DAM) CNG channels. Collectively, my thesis work contributes to a deeper understanding of the structural determinants of CNG channel properties, provides a detailed dissection of the CNG channel gating mechanism, demonstrates a potential CNG channel pathogenic mechanism, and calls for an interdisciplinary reevaluation of CNG channel DAMs.