Following determination of the first crystal structure of the reaction center of Rhodopseudomonas viridis, a membrane protein, by X-ray crystal structure analysis at 3.0 Å resolution, 18 X-ray crystal structures and two electron crystal structures of membrane proteins have been obtained at higher than 3.5 Å resolution. Besides these integral membrane protein structures, three crystal structures of water-soluble proteins, which can enter membranes, have been determined by X-ray crystallography at high resolution. The structural features of membrane proteins have been summarized by inspecting these crystal structures. The polypeptide chain crosses the membrane in a helical conformation or a β-strand. The central +10 Å region of the transmembrane α-helix is dominated by hydrophobic residues. On both sides of the central region are concentrated polar aromatic residues. Charged residues are dominant around +15 Å to +20 Å. All the transmembrane β-structures are found in pore-forming proteins. The central region of the transmembrane β-structure is amphipathic with hydrophobic residues on the membrane exposed side. The distribution of amino acid residues on the membrane exposed surface of the transmembrane β-structure is similar to that of the transmembrane α-helix. α-Helices anchoring the membrane surface region are amphipathic with hydrophobic residues inside and hydrophilic residues outside.