Photosynthesis in plants is a very complicated process, utilizing two photosystems in series to carry out the very energy-demanding process of oxidizing water to molecular oxygen and reducing carbon dioxide to organic compounds. The first photosynthetic organisms, living more than 3.4, perhaps even 3.8 Ga, i.e. American billion (109) years ago, carried out a simpler process, without oxygen production and with only one photosystem. A great variety of such one-photosystem photosynthesizers are living even today, and by comparing them, and from chemical fossils, researchers are trying to piece together a picture of the course of the earliest evolution of photosynthesis. Chlorophyll a probably preceded bacteriochlorophyll a as a main pigment for conversion of light into life energy. The process of carbon dioxide assimilation, today taking place mainly in conjunction with photosynthesis, is even older than photosynthesis itself. Oxygenic photosynthesis, i.e. photosynthetic production of molecular oxygen, first appeared in ancestors of present-day cyanobacteria more than 2.7, perhaps already 3.7 Ga ago. Cyanobacteria entered into close association with other organisms more than 1.2 Ga ago, and chloroplasts in green algae and green plants as well as those in algae on the "red" line of evolution (red algae, cryptophytes, diatoms, brown algae, yellow-green algae and others) stem from a single early event of endosymbiotic uptake of a cyanobacterium into a heterotrophic organism. Only ecologically unimportant exceptions from this rule have been found. The chloroplasts on the "red line", excepting those of red algae, stem from a single event of secondary endosymbiosis, in which a red alga was taken up into another organism. There are also examples of tertiary (third level) endosymbiotic events. Thylakoids in land plants are partially appressed and forming grana, while those of, e.g., red algae do not have this structure, and this difference can be explained by the different spectra of ambient light. At the end of the chapter a brief review is given of the evolution of the assimilation of carbon dioxide, the adaptation to terrestrial life, and the impact of photosynthesis on the terrestrial environment.