Interest in cancer immunotherapy has grown in recent years due to its potential for significant and durable therapeutic responses. Immune checkpoint blockade has emerged as an immunotherapy as a single agent but has even greater appeal when it is used in combination with other immunostimulatory approaches. However, the dosing of checkpoint blockade and its combinatorial use with other immunotherapies has been limited by systemic immune-related adverse side effects. One way to overcome these adverse effects is to deliver the therapeutic agents specifically to the tumor microenvironment. Salmonella enterica Typhimurium (S. Typhimurium) has been studied for cancer therapy due to its genetic manipulability and tumor-targeting propensity, and in this thesis, the potential of S. Typhimurium as a tumor-targeting immunotherapy vector was investigated. Functional antagonistic single chain antibodies (scFvs) against the immune checkpoints CTLA-4 and PD-L1 were isolated from an immunized chicken library and engineered for secretion from S. Typhimurium. The inherent anti-tumor properties and tumor-targeting capability of S. Typhimurium were then tested in transplanted primary and metastatic tumor models as well as a genetically engineered autochthonous BALB-neuT breast cancer model. In each of these models, S. Typhimurium demonstrated native anti-tumor efficacy; however the bacteria did not adequately colonize the autochthonous tumors of the BALB-neuT model. Disruption of tumor vasculature by treating BALB-neuT mice with a vasculature disrupting agent (VDA) improved the colonization of autochthonous tumors over 1000-fold to levels similar to those observed for transplanted tumors. Subsequent comparison of the tumor targeting capability and efficacy of S. Typhimurium engineered to secrete the antagonistic ?PD-L1 (scFv) versus a control strain showed that secretion of the scFv may further improve the colonization of autochthonous tumors, leading to a greater reduction in tumor burden of treated mice. These findings provide a proof of principle for the expression and delivery of functional immunotherapeutic single chain antibodies using S. Typhimurium, demonstrate S. Typhimurium's native tumoricidal activity independent of tumor-targeting, illustrate the importance of clinically representative tumor models when studying bacterial cancer therapy, and demonstrate the potential of VDA treatment to improve bacterial tumor-targeting. Collectively, this work illustrates S. Typhimurium's promise as a tumor-targeting immunotherapy vector.