Cells, especially eukaryotic cells, have prominent delimiting thin films which are termed \'cell membranes\'. These membranes implement a vast array of cellular functionality, from retention and seperation, to the transport of solutes.

The primary roles of the cell membrane are to:
- Form the boundary of a cell
- Provide a reaction and attachment surface
- Permit transport of substances in and out of the cell
- Hold equipment that senses extracellular parameters
- Permit cell-to-cell signalling (intercellular signalling).

The structure of cell membranes was discovered by Signer and Nicolson to be that of the \'Fluid Mosaic Model\'. This model is generally accepted as the membrane structure. In this article, we discuss only this Fluid Mosaic Model, as it is universally recognised as accurate.

The Fluid phase of the membrane is made up of Membrane Lipids. These make up a lipid bilayer, first proposed by Gorter and Grendel. The bilipid layer means that there are two layers of lipid, each opposing one another, with the hydrophobic \'heads\' facing the inside and outside of the cell, they hydrophillic \'tails\' making up the internal meeting points. These lipids include Phospholipds, Glycolipids, and Sterols.

These lipids are both fluid (i.e. they can move around the membrane), and asymetrically distributed between the two layers.