The molecular chaperone heat shock protein 90 (HSP90) is crucial for the function of numerous oncoproteins in cancer cells, making it a promising target for therapeutic intervention. Recent clinical trials of HSP90 inhibitors have shown early promise, but their optimal use depends on understanding the complex regulation of HSP90 and its role in both neoplastic and normal cellular physiology. HSP90 forms a dynamic complex with co-chaperones and is regulated by post-translational modifications, including phosphorylation, acetylation, and S-nitrosylation. These modifications affect HSP90's chaperone function and its interactions with various proteins. HSP90 also plays a role in nuclear events such as transcription regulation, chromatin remodeling, and DNA mutation. Clinical trials of HSP90 inhibitors have shown efficacy in various cancers, particularly those driven by HSP90 clients like HER2 and BCR-ABL. However, resistance mechanisms, such as reduced NQO1 expression, have been identified. Alternative methods for targeting HSP90, such as inhibiting co-chaperone interactions, are being explored. The optimal development and application of HSP90-targeted therapeutics will require a comprehensive understanding of HSP90's role in cancer and its regulation.The molecular chaperone heat shock protein 90 (HSP90) is crucial for the function of numerous oncoproteins in cancer cells, making it a promising target for therapeutic intervention. Recent clinical trials of HSP90 inhibitors have shown early promise, but their optimal use depends on understanding the complex regulation of HSP90 and its role in both neoplastic and normal cellular physiology. HSP90 forms a dynamic complex with co-chaperones and is regulated by post-translational modifications, including phosphorylation, acetylation, and S-nitrosylation. These modifications affect HSP90's chaperone function and its interactions with various proteins. HSP90 also plays a role in nuclear events such as transcription regulation, chromatin remodeling, and DNA mutation. Clinical trials of HSP90 inhibitors have shown efficacy in various cancers, particularly those driven by HSP90 clients like HER2 and BCR-ABL. However, resistance mechanisms, such as reduced NQO1 expression, have been identified. Alternative methods for targeting HSP90, such as inhibiting co-chaperone interactions, are being explored. The optimal development and application of HSP90-targeted therapeutics will require a comprehensive understanding of HSP90's role in cancer and its regulation.