A plant canopy, a collection of leaves, is an ecosystem-level unit of photosynthesis that assimilates carbon dioxide and exchanges other gases and energy with the atmosphere in a manner highly sensitive to ambient conditions including atmospheric carbon dioxide and water vapor concentrations, light and temperature, and soil resource availability. In addition to providing carbon skeletons and chemical energy for most of the living organisms, these key  canopy functions affect global climate through modification of atmospheric carbon dioxide concentration and through altering surface albedo. This interaction, the climate-carbon cycle feedback, is one of the most uncertain processes for projection of future global climate. This book describes our current knowledge of canopy photosynthesis that has accumulated over the last hundred years since the pioneering study of P. Boysen Jensen. The book provides a comprehensive analysis of plant canopy physiology, ecology and physics with emphasis on predictive modeling techniques. The book is divided into five parts covering hierarchy of canopy processes in time and space. Two chapters in Part 1 discuss the basic physical processes on light attenuation and energy transfer in plant canopies, while three chapters in Part 2 deal with the principle mechanisms of leaf gas-exchange regulation and the patterns and mechanisms of variations in leaf traits. Three chapters in Part 3 focus on whole-plant processes in plant canopies. Part 4 (in four chapters) describes how vegetation functions are assessed by modeling, eddy-covariance techniques, and remote sensing and forest inventory. Finally, three chapters in Part 5 discuss the relationships between canopy photosynthesis and other vegetation processes in plant stands.