Photosystem I is an integral component of the thylakoid membrane which catalyzes the photoreduction of ferredoxin using plastocyanin or cytochrome c as electron donor. In higher plants, the photosystem I complex is composed of eight protein subunits, chlorophyll a, carotenoids, phylloquinone and bound iron sulfur clusters. The molecular biology and biochemistry of the complex are discussed in relation to the structure and function of the individual components. The mechanisms involved in the assembly of the components into a functional complex are also discussed.