Plastids are a class of membrane-bound organelles found in plant and algal cells. In plants, they may differentiate into several forms, depending upon which function they need to play in the cell. Undifferentiated plastids (proplastids) may develop into any of the following plastids:

• Amyloplasts: for starch storage
• Chloroplasts: for photosynthesis
• Etioplasts : chloroplasts that have not been exposed to light
• Elaioplasts: for storing fat
• Chromoplasts: for pigment synthesis and storage
• Leucoplasts: for monoterpenes synthesis

In algae, the term leucoplast (leukoplast) is used for all unpigmented plastids. Its function differs from the leukoplasts in plants. Etioplast, amyloplast and chromoplast are plant-specific and do not occur in algae. Algal plastids may also differ from plant plastids in that they contain pyrenoids.

Plastids are thought to have originated from endosymbiotic cyanobacteria. Due to a split-up into three evolutionary lineages, the plastids are named differently: chloroplasts in green algae and plants, rhodoplasts in red algae and cyanelles in the glaucophytes. The plastids differ by their pigmentation, but also in ultrastructure. The chloroplasts e.g. have lost all phycobilisomes , the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but - only in plants and in closely related green algae - contain stroma and grana thylakoids. The glaucocystophycean plastid - in contrast to the chloroplasts and the rhodoplasts - is still surrounded by a remains of the cyanobacterial cell wall. All these primary plastids are surrounded by two membranes.

Complex plastids originate from a secondary endosymbiosis (i.e. a eukaryote engulfed a red or green alga and reduced it to a plastid) and are surrounded by more than two membranes. Algae with complex plastids derived from a secondary endosymbiosis event with a red alga are the heterokonts, haptophytes, cryptomonads, and most dinoflagellates (= rhodoplasts). Those with endosymbioses with green algae are the euglenids and the chlorarachniophytes (= chloroplasts). The Apicomplexa (e.g. the cause of the Malaria disease, Plasmodium) also have complex plastids, which stopped photosynthesis and turned into leucoplasts. These plastids are extremely reduced and it is not yet clear whether they derived from red or green algae.

Kleptoplastids are found in some dinoflagellates. The dinoflagellates take up algae as food. They keep the plastid of the digested alga to profit from the photosynthesis, but this plastid is also digested after a while.