The heterogeneous vascular distribution and blood flow in solid tumors create areas of low oxygen partial pressure or hypoxic areas. Hypoxia acts as a stressor that perpetuates the malignant phenotype by increasing genomic instability, altering gene expression, and the tumor's metabolic microenvironment. The heterodimeric transcription factor, hypoxia inducible factor, HIF-1 (Hypoxia-Inducible Factor-1), allows tumor cells to adapt to their microenvironment by stimulating the expression of a large number of hypoxia-related genes. Hypoxia has been shown to reduce chemotherapeutic efficacy by causing cells within hypoxic regions to cycle more slowly, and by providing a selection mechanism for cells with reduced susceptibility for apoptosis. Due to limited drug penetration within solid tumors, hypoxic regions (especially those located furthest from blood vessels) are often protected from the cytotoxic effects of chemotherapeutic agents further reducing drug efficacy. Given the contribution of tumor hypoxia to tumor progression and drug resistance, a number of hypoxia-targeted therapeutics are under development. These therapeutics ranges from hypoxic cytotoxins, such as tirapazamine and the pro-drug AQ4N, to the use of obligate anaerobic bacterial spores. Development of methodologies to characterize causes of drug resistance related to the tumor microenvironment has considerable potential to improve the outcome following systemic treatment of solid tumors.