Most functional entities within cells are composed of protein complexes. The actual challenge for structural biologists is to purify these complexes, or at least functional subcomplexes, in sufficiently large amounts for structural characterization. One major technique for assembling complexes is coexpression of complex components in the same host cell, as it combines the advantages of in vivo and in vitro techniques. Several hosts can be used for coexpression, including Escherichia coli, insect and mammalian cells. Strategies that enable high throughput combinatorial coexpression of many proteins are discussed. The simplicity, versatility, cost effectiveness and success of E. coli can only be rivalled by the sophistication of the eukaryotic cells, providing more complicated posttranslational processing of the complex components sometimes required for complex formation. The technique of coexpression can easily be integrated in semiautomated approaches for the high throughput characterization and structure determination of protein complexes.