Cancer remains a leading cause of death in the United States. The most common treatment options include chemotherapy, but poor solubility, adverse side effects and differential drug sensitivity hamper clinical applications. Current chemotherapy generally aims to deliver drugs at the limit of toxicity, assuming that higher dosage increases efficacy, with little attention paid to potential benefits of tunable release. Growing evidence suggests that releasing drugs at a constant rate will be as effective as a single bolus dose. To test this hypothesis, it is critical to develop drug delivery systems that fine-tune drug release and elucidate the impact of tunable drug release rates on chemotherapeutic efficacy. Block copolymer micelles, spherical nanoassemblies with a core-shell structure, are widely used in recent research. Micelles for this study were engineered to release a model drug (doxorubicin: DOX) at differential rates under acidic conditions, corresponding to tumor tissue (pH < 7). Three specific aims were pursued: to develop drug carriers capable of tuning drug release rates; to determine activity of developed carriers in vitro; and to elucidate effects of tunable drug release rates in vivo. Block copolymers with covalently linked DOX were synthesized and self-associated, forming micelles. Drug binding linkers (glycine, aminobenzoate, or hydrazide) were used to tune release of DOX. Micelles were characterized to determine physicochemical properties such as particle size, drug entrapment yields, and drug release parameters. Characterization revealed that drug release profiles were modulated by interchanging drug binding linkers. Micelles were evaluated in vitro to elucidate the effect of tunable drug release. Micelles delivered drugs at a slower, prolonged rate compared to free DOX. Cytotoxicity and cellular internalization analysis revealed that by slowing release rates, micelles kill cells more efficiently. Biodistribution studies showed that micelles decrease DOX accumulation in peripheral tissue while increasing the maximum tolerated dose. Antitumor activity studies verified that micelles with slower release rates better suppressed tumor growth. This further confirms that release rates play a key role in chemotherapeutic efficacy. Therefore, this thesis provides better insights into the effects of tunable drug release in tumors, leading the way for improved chemotherapy treatments in the future.