Heat shock protein 90 (Hsp90) is an important target in cancer because of its role in maintaining transformation and has recently become the focus of several drug discovery and development efforts. While compounds with different modes of action are known, the focus of this review is on those classes of compounds which inhibit Hsp90 by binding to the N-terminal ATP pocket. These include natural product inhibitors such as geldanamycin and radicicol and synthetic inhibitors comprised of purines, pyrazoles, isoxazoles and other scaffolds. The synthetic inhibitors have been discovered either by structure-based design, high throughput screening and more recently using fragment-based design and virtual screening techniques. This review will discuss the discovery of these different classes, as well as their development as potential clinical agents.