All animals, including human beings, depend on plants for their food. The green plants make or rather synthesise the food they need through photosynthesis and are therefore called autotrophs. Green plants carry out ‘photosynthesis’, a physicochemical process which uses light energy to drive the synthesis of organic compounds. Photosynthesis is important due to two reasons: it forms the basis for all known food chains on earth. It is also responsible for the release of oxygen into the atmosphere by green plants. Sun is the ultimate source of energy, therefore, all living forms on earth depend on sunlight for their energy. This chapter focusses on the structure of the photosynthetic machinery and the various reactions that transform light energy into chemical energy.