Complicated system of membranes within eukaryotic cells involved in the production and transportation of proteins, which are targeted to the the outside of the cell or to the plasma membrane, or involved in the production and modification of hydrophobic substances.

Proteins for the extracellular space and for the plasma membrane of the cell are produced by ribosomes at the surface of the ER and directly transported into this membrane system. There they are partially modified by the addition of carbohydrate chains, before they are transported via the golgi apparatus to their destinations.