Integral membrane proteins are a major challenge for protein-structure prediction. It is estimated that about a third of genes code for membrane proteins (1), and yet high-resolution structures are known for only a handful of these. Furthermore, technical problems of protein expression and crystallization suggest that an explosive expansion in the number of membrane-protein-structure determinations is still in the future. In this chapter, attention is restricted to the major class of membrane proteins, i.e., those formed by bundles of transmembrane (TM) α-helices. Prediction methods also exist for those membrane proteins (e.g., porins and some bacterial toxins) that are formed by β-barrels (Kay Diederichs, personal communication; also see website: http://loop.biologie.uni-konstanz.de/~kay/om_topo_predict2.html). However, these methods are not applicable to the majority of membrane proteins and so are not discussed here.