Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allen et al. 1987; Chang et al. 1991; Deisenhofer & Michel, 1989; Ermler et al. 1994); porins (Cowan et al. 1992; Schirmer et al. 1995; Weiss et al. 1991); prostaglandin H2 synthase (Picot et al. 1994); light harvesting complex (McDermott et al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Henderson et al. 1990); light harvesting complex (Kühlbrandt et al. 1994)].