A double membrane is a membrane structure consisting of two phospholipid bilayers arranged back to back. Double membranes define cellular compartments known as organelles. The space delimited by the two membranes makes up the organelle's lumen, which is distinct from the cytosol.

One of the most prominent double membrane structures in eukaryotic cells is the nuclear envelope, which encloses the cell's genetic material. Other well-known organelles that possess double membranes include mitochondria and chloroplasts. In mitochondria, the double membrane structure serves to generate a proton gradient for the production of ATP through oxidative phosphorylation. In chloroplasts, the double membrane is involved in capturing and converting sunlight into chemical energy during photosynthesis.

Double membranes play essential roles in cellular compartmentalization, transport of molecules, and the maintenance of specific chemical environments within organelles, making them critical components of eukaryotic cell structure and function.