Intratumoral injection of oncolytic viruses provides a direct means of tumor cell elimination for inoperable tumors. Unfortunately, oncolytic vectors based on human adenovirus (HAdV) typically do not spread efficiently throughout the tumor mass, reducing the efficacy of treatment. In this thesis, I explore the efficacy of conditionally replicating HAdV vectors expressing either the p14 Fusion Associated Small Transmembrane (FAST) protein (CRAdFAST) or p14 FAST protein in combination with the adenovirus death protein (CRAdFAST-ADP). The p14 FAST protein mediates cell-cell fusion, which may enhance spread of the virus-mediated, tumor cell-killing effect, while ADP aids in cell lysis and HAdV spread at late times in infection. I first explored the efficacy of CRAdFAST in the 4T1 immune competent mouse model of cancer. Treatment with CRAdFAST resulted in enhanced cell death compared to vector lacking the p14 FAST gene in vitro, but did not reduce the tumor growth rate in vivo. The 4T1 model was significantly resistant to HAdV infection and propagation, so I next explored CRAdFAST efficacy in human A549 cell culture and a xenograft mouse model of cancer. In the human A549 lung adenocarcinoma model of cancer, CRAdFAST showed significantly improved oncolytic efficacy in vitro and in vivo. In an A549 xenograft tumor model in vivo, CRAdFAST induced tumor cell fusion which led to the formation of large acellular regions within the tumor, and significantly reduced the tumor growth rate compared to control vector. Finally, to assess the use of a newly constructed CRAdFAST vector co-expressing the adenovirus death protein (ADP), a new model was explored comprised of CMT-64.6 mouse lung carcinoma cells which are syngeneic with Balb/C mice. This model was significantly more sensitive to HAdV infection and CRAdFAST induced fusion than the 4T1 cell line. In this model, expression of ADP and p14 FAST from a CRAdFAST-like vector (CRAdFAST-ADP) resulted in significant oncolytic synergy in vitro but not in vivo. My results indicate that expression of p14 FAST protein, and potentially ADP, from an oncolytic HAdV can improve vector efficacy for the treatment of cancer, but improved in vivo models will be required to analyze the full preclinical potential of these oncolytic HAdV vectors.