The transcription factor hypoxia-inducible factor-1 (HIF-1) regulates the expression of more than 100 genes involved in cellular adaptation and survival under hypoxic stress. Activation of HIF-1 is associated with numerous physiological and pathological processes that include tumorigenesis, vascular remodelling, inflammation, and hypoxia/ischemia-related tissue damage. Experimental data support the concept that modulation of Reactive Oxygen Species (ROS) levels have an important impact on the hypoxic response mediated by HIF-1 alpha. However, ROS generation, the exact kinetics and conditions of ROS production and their specific relevance to HIF-l alpha activation are issue still to be clarified. Clinical studies suggested that HIF-1 activation correlates directly with advanced disease stages and treatment resistance among cancer patients. Preclinical studies support the inhibition of HIF-1 as a major molecular target for anti-tumour drug discovery. Considerable effort is underway to identify therapeutically useful molecule HIF-1 inhibitors. Most of the compounds discovered to inhibit HIF-1 are natural products or synthetic compounds with structures that are based on natural product leads. Natural products have also served a vital role as molecular probes to elucidate the pathways that regulate HIF-1 activity. Many of the substances found to inhibit HIF-I are non-druggable compounds that are too cytotoxic to serve as drug leads. The application of high-throughput screening methods, complementary molecular-targeted assays, and structurally diverse chemical libraries hold promise for the discovery of therapeutically useful HIF-1 inhibitors.