Heat shock protein 90 (Hsp90) is an important member of the chaperone protein family and it is involved in stabilization, regulation, and maintenance of oncogenic client proteins with co-chaperones. Co-chaperones regulate the ATPase activity of Hsp90 and its interactions with oncogenic client proteins. Therefore, Hsp90 and its co-chaperones have become significant therapeutic targets for cancer treatment. Many chemical compounds have been evaluated for Hsp90 inhibition as well as significant results were obtained in clinical trials. In this paper, we emphasize on the key roles of Hsp90 and its co-chaperones in tumorigenesis and overview therapeutic strategies of Hsp90 inhibition in oncology.