Targeted delivery of drugs to tumors is critical for optimal implementation of chemotherapy. Targeted drug delivery can be achieved by encapsulating drugs in nanometer-sized drug carriers. Consequently, a significant amount of preclinical research is focused on the development of nanoparticle-based chemotherapeutics. In spite of many key advances, these systems still have some major limitations. These stem from extensive distribution of nanoparticles to off-target sites, and the presence of several resistance mechanisms within the tumor. The goal of this thesis was to investigate, and potentially improve, the various steps involved in the delivery of nanoparticles to solid tumors. Using a pharmacokinetic modeling approach, the effect of physicochemical properties of nanoparticles and vascular permeability of the tumor on the systemic distribution of nanoparticles was studied. Transport across compartments was calculated using equations for transvascular convection and diffusion of nanoparticles. The results of the modeling studies suggested that optimal drug targeting would be observed only in an intermediate particle size range. At a particle size below the vascular pore size of the toxicity compartment, nanoparticles extensively accumulated in that compartment. This reduced the fraction available for deposition into the tumor. At a particle size above the vascular pore size of the tumor, accumulation of nanoparticles in the tumor was not possible. Most of the drug was either eliminated in the form of nanoparticles or released in the central compartment. Hence, the lower and upper bounds of the optimal particle size range were determined by the vascular pore size of the toxicity and target compartments, respectively. With a decrease in drug release rate, the average number of visits of the drug in the target compartment increased. Hence, decreasing the rate of drug release could favorably affect the drug targeting index. Maximal benefit with this strategy was also experienced in an intermediate particle size range, which was dictated by vascular pore sizes of the toxicity and target compartments. Due to leaky tumor blood vessels and prothrombogenic tumor microenvironment, fibrinogen gets deposited in the tumor and is converted to cross-linked fibrin. Using a series of immunohistochemistry analysis and in vitro diffusion studies, it was established that most human solid tumors are characterized by the deposition of a significant amount of fibrin and that the presence of fibrin retarded the mobility of nanoparticles. However, no previous studies have analyzed the influence of fibrin on the intratumoral distribution of nanoparticles. The central hypothesis of this chapter was that degrading fibrin using a fibrinolytic enzyme [tissue plasminogen activator (tPA)] will improve the intratumoral distribution of nanoparticles and their chemotherapeutic efficacy. In an A549 orthotopic lung cancer model, co-administration of tPA improved the anti-cancer efficacy of paclitaxel nanoparticles (p<0.05, on Day 73 after commencement of treatment). In this chronic study, the average tumor bioluminescence in animals treated with the combination therapy was lower than the other groups for >2 months. In a B16F10 mouse melanoma model the combination therapy reduced the rate of tumor growth by ~1.5 fold relative to that with just paclitaxel nanoparticles (p=0.08). Further, immunohistochemistry revealed that administration of tPA resulted in the decompression of tumor blood vessels. Using Power Doppler ultrasound, we established that treatment with tPA led to a 3-4 fold improvement in tumor perfusion (p<0.05, after second dose). All these results suggest that fibrinolytic therapy may lead to an enhanced intratumoral distribution of nanoparticles. Tylocrebrine is a potent anti-cancer drug but has a narrow therapeutic index. Extensive distribution to the central nervous system and reduced cell uptake in the acidic tumor microenvironment limited the clinical translation of the drug. To address both issues, tylocrebrine was encapsulated in polymeric nanoparticles targeted to the epidermal growth factor receptor (EGFR). In vitro studies in human cancer cell lines showed that decrease in the extracellular pH led to a 3-fold decrease in drug uptake (p<0.05) and ~2-10 fold reduction in drug potency. However, when encapsulated in targeted nanoparticles, its potency was less affected by extracellular pH. Pharmacokinetic studies in mice revealed that the drug rapidly accumulated in the brain, and brain accumulation could be reduced by encapsulation in nanoparticles (~2-fold; p<0.05, compared to free drug at 0.5 h post dose). When delivered in targeted nanoparticles, the tumor accumulation and retention of the drug was also improved (p<0.05, compared to free drug at 4h post dose). In a xenograft mouse model of human epidermoid cancer, treatment with tylocrebrine solution retarded tumor growth. However, tumor inhibition was found to be more significant with EGFR targeted nanoparticles (p<0.05, compared to saline and non-targeted nanoparticle treated animals). This effect was likely due to the improved tumor accumulation and higher potency of targeted nanoparticles. In conclusion, this thesis presents an analysis of the three steps of nanoparticle transport from the site of administration to the site of action. These steps include systemic distribution, intratumoral distribution and cell uptake. Improving the efficiency of each of these steps can improve the overall efficacy of chemotherapy.