This chapter considers how cell membranes control the traffic of inorganic ions and small, water-soluble molecules into and out of the cell and its membrane-enclosed organelles. It outlines some of the general principles that guide the passage of ions and small molecules through cell membranes. The chapter examines the two main classes of membrane proteins that mediate this transfer:transporters and channels. The smaller the molecule and, more importantly, the fewer its favorable interactions with water the more rapidly the molecule diffuses across the bilayer. The movement of the ions across cell membranes plays an essential part in many biological processes, but is perhaps most striking in the production of ATP by all cells, and in communication by nerve cells. Transfer of nutrients, metabolites, and inorganic ions across cell membranes depends on membrane transport proteins. Channel proteins form pores across the lipid bilayer through which solutes can passively diffuse.