Photosystem I is a large membrane protein complex that catalyzes the first step of light reactions in photosynthesis. It can be regarded as a solar energy converter that captures the light from the sun through a large core-antenna system of chlorophylls and carotenoids. It then transfers the excitation energy into the center of the complex, where this electronic energy is used to catalyze the light-driven transmembrane electron transfer from plastocyanin to ferredoxin. Photosystem I of cyanobacterial origin consists of 12 protein subunits, to which 127 cofactors are non-covalently bound. This chapter describes the structure and function of cyanobacterial Photosystem I, as revealed from the X-ray structure at 2.5 Å resolution.