The structure of proteins that are difficult to crystallize can often be solved by forming a noncovalent complex with a helper protein--a crystallization "chaperone." Although several such applications have been described to date, their handling usually is still very laborious. A valuable addition to the present repertoire of binding proteins is the recently developed designed ankyrin repeat protein (DARPin) technology. DARPins are built based on the natural ankyrin repeat protein fold with randomized surface residue positions allowing specific binding to virtually any target protein. The broad potential of these binding proteins for X-ray crystallography is illustrated by five cocrystal structures that have been determined recently comprising target proteins from distinct families, namely a sugar binding protein, two kinases, a caspase, and a membrane protein. This article reviews the opportunities of this technology for structural biology and the structural aspects of the DARPin-protein complexes.