The endomembrane system is the system of internal membranes within eukaryotic cells that divide the cell into functional and structural compartments, or organelles. Prokaryotes do not have an endomembrane system and thus lack most organelles.

The endomembrane system also provides a transport system, for moving molecules through the interior of the cell, as well as interactive surfaces for lipid and protein synthesis. The membranes that make up the endomembrane system are made of a lipid bilayer, with proteins attached to either side or traversing them.

The following organelles are part of the endomembrane system:

• The plasma membrane is a lipid bilayer membrane that separates the cell from its environment and regulates the transport of molecules and signals into and out of the cell.
• The nuclear envelope is the membrane around the nucleus of the cell.
• The endoplasmic reticulum is a synthesis and transport organelle [the endoplasmic reticulum is an extension of the nuclear envelope].
• The Golgi apparatus acts as the packaging and delivery system for molecules.
• Lysosomes are the "digestive" units of the cell. They utilize enzymes to break down macromolecules and also act as a waste disposal system.
• Vacuoles act as storage units in some cells.
• Vesicles are small membrane-enclosed transport units that can transfer molecules between different compartments.