This chapter covers the identification and characterization of the novel Hsp90 inhibitor NVP-AUY922, which entered clinical trials in July 2007. The program leading to the identification of NVP-AUY922 (VER-52296) used complementary hit ID technologies such as HTS and fragment-based screening, relied heavily on structure-based design and required the design of new assays and probes to drive the compound optimization. In vivo characterization in tumor-bearing animals assisted in compound selection for pre-clinical studies.