Plastids are semiautonomous organelles with a wide structural and functional diversity and unique biochemical pathways. As such, they are able to transcribe and translate the information present in their own genome but are strongly dependent on imported proteins that are encoded in the nuclear genome and translated in the cytoplasm. Plastids are present in every plant cell, with very few exceptions (such as the highly specialized male sexual cells), and their structural and functional diversity reflects their role in different cell types. According to their devel- opmental stage, we distinguish them as juvenile (proplas- tids), differentiating, mature, and senescent. Meristematic cells contain proplastids, which ensure the continuity of plastids from generation to generation and are capable of considerable structural and metabolic plasticity to develop into various types of plastids that remain interconvertible. When leaves are grown in darkness, proplastids differen- tiate into etioplasts, which can be converted into chloro- plasts under illumination. The metabolism of these various types of plastids is linked to the function of the tissue in which they are found. For instance, whereas the chief function of illuminated leaves is the assimilation of CO2 by chloroplasts, root plas- tids are mainly involved in the assimilation of inorganic nitrogen. Amyloplasts, which contain large starch grains, behave as storage reservoirs in stems, roots, and tubers. Chromoplasts synthesize large amounts of carotenoids and are present in petals, fruits, and even roots. The intercon- versions between these different plastids are accompanied by dramatic changes, including the development or regres- sion of internal membrane systems (e.g. thylakoids and prolamellar bodies) and the acquisition of specific enzy- matic equipment reflecting specialized metabolism. How- ever, at all stages of these transformations, the two limiting envelope membranes remain apparently unchanged.