This book summarizes the advances made in the last decade in the biophysics, biochemistry, and molecular biology of the enzyme known as Photosystem I, the light-induced plastocyanin: ferredoxin oxidoreductase. This volume is a unique compilation of chapters that include information on molecular architecture, protein-pigment interactions, excitation and electron transfer dynamics, protein-cofactor interactions, kinetics of electron transfer and bioassembly of proteins and cofactors. The volume begins with a series of historical perspectives that provide a solid background to the field, and ends with information on modelling of light-harvesting and electron transfer reactions, and the evolution of the reaction center. Particular attention is paid to spectroscopy, including the theory of the measurement and the interpretation of the data.