Plastids are a group of organelles present in the cells of all higher and lower plants, including algae, which function in a variety of different ways to enable plants to grow and function. Although different types of plastids which are found in different types of cells have modified roles, according to the type of cell in which they reside, the foremost function of plastids is carrying out the process of photosynthesis. Photosynthesis is a fundamental feature of plants and is facilitated by the presence of green, pigmented chloroplasts within plant cells. Indeed, photosynthesis is a defining feature of plants and enables them to fix carbon from the gaseous carbon dioxide in the air and synthesise a variety of complex organic molecules which allows them to increase in stature and mass. Photosynthesis is carried out by chloroplasts, which by virtue of containing the green pigment chlorophyll, defines the phenotype of green plants. Photosynthetic Eukaryotes have increased in their complexity dramatically since the first land plants, termed Embryophytes, evolved from freshwater multicellular green algae, around 450 million years ago. The current-day group of algae that are most closely related to these ancient algae are the Chlorophytes (Fig. 1.1). From these have evolved the lower plants, which includes the liverworts, mosses, hornworts and ferns (Fig. 1.1). Subsequently, the Gymnosperms and then the flowering plants, the Angiosperms, evolved and the Angiosperms, in particular, have been highly successful in conquering the planet such that much of the Earth is covered in green swathes of vegetation containing countless numbers of photosynthetic chloroplasts within their cells.