The study of the large, unactivated form of steroid receptors has led to the discovery of an hsp90/hsp70-based multicomponent protein folding system(s). For steroid receptors, the hsp90 chaperone system determines both repression of transcriptional activity in the absence of hormone and the proper folding of the hormone binding domain to produce the steroid binding conformation. Like steroid receptors, a number of other regulators of transcription and some protein kinases are now known to be associated with hsp90. Given the abundance of the proteins comprising the hsp90 chaperone system and the apparent ubiquity of the system in the animal and plant kingdoms, this system is thought to serve a fundamental role for protein folding, function and possibly trafficking within the cytoplasm and nucleus. In this chapter, we discuss the work on steroid receptor heterocomplex composition that has led to the discovery of new chaperone proteins and we summarize the mechanistic information developed in cell-free studies of receptor heterocomplex assembly.