The advantages of non-viral gene transfer systems are safety and low immunogenicity, therefore they are well suited for use as vectors in gene therapy. The main disadvantage, namely their low gene-transfer-efficiency, can be improved through the development of systemic gene transfer systems using targeted vectors with high specificity and gene transfer efficiency. The intravenous application of PEI22lin/DNS complexes leads to a high gene expression in the lung, but with high toxicity. This observation can be explained by the positive surface charge of the DNS complexes and the uncomplexed free PEI, which leads to aggregation of erythrocytes. DNS complexes can be isolated from free uncomplexed PEI by gel filtration. The systemic application of gel filtrated PEI22lin complexes to non-tumor bearing mice resulted in reduced toxicity however there was a decreased in gene expression compared to non-filtrated complexes. The same experiment was performed on tumor bearing mice and again reduced toxicity was observed and interestingly slightly higher gene expression found in the tumor compared to the non-filtrated complexes. Shielding the positive surface charge of the PEI22lin complexes by transferrin led to increased gene expression in the tumor with reduced expression in the lung and other organs. The improved tumor targeted gene expression was associated with reduced systemic toxicity. Tumor targeted gene expression appears to be dependent on the tumor model as this observation was only found in neuro2A neuroblastoma tumor model in A/J mice and not in B16F10 melanoma tumor models of C57BL/6 mice and CT26 colon carcinoma tumor models of BALB/c mice. To enhance the intracellular efficiency of the vectors, the endosomolytic active peptide melittin was incorporated into the transferrin targeted complexes. This led to a further increase in gene expression in the Neuro2A tumor models in A/J mice. For the local gene transfer, electroporation proved to be an easy to handle method to obtain a high gene expression in tissue. The non-invasive kaliper electrode was suitable for gene transfer to both muscle and tumor. The applied voltage showed to be the most important parameter in expression. The use of electroporation for intratumoral transfer of the therapeutic gene encoding for the cytokine TNF was unsuccessful. However, systemic application of the TNF-α gene in transferrin targeted complexes in combination with the intraperitoneal application of the chemotherapeutic Doxil® showed a clear synergistic effect. A significant delay in the tumor growth and in some cases a complete regression of the tumor was observed. The enzyme cytochrome P450 metabolizes the non toxic prodrug cyclophosphamide (CPA) into the cytotoxic drug. Electroporation of the cytochrome P450 gene into the tumor lead to its localized protein expression. When followed by the intraperitoneal application of CPA, a significant delay in the tumor growth of the human hepatocellular carcinoma Huh7 was observed in SCID mice. When applied to the Neuro2A tumor model in A/J mice, this application scheme showed a complete tumor regression in two animals. Furthermore the systemic application of the P450 gene in transferrin targeted complexes containing melittin in combination with CPA led to a strong delay in the tumor growth. In summary, this work describes a new anti-cancer strategy using the combination of chemotherapeutics and non-viral gene delivery resulting in a synergistic therapeutic effect in vivo. This promising strategy will be more effective with the improvement of non-viral gene delivery systems which have better targeted gene expression with lower toxicity.