The use of various synthetic lipids and polymers to deliver DNA for gene therapy applications has been the subject of intense examination for the last 15 years. Our understanding of the processes involved in the delivery of DNA, although still limited, can be described in terms of specific physical and chemical barriers encountered along the delivery pathway. Successful engagement of this pathway involves avoiding inactivation in the extracellular compartment and initial favorable interactions with the cell surface. Internalization of the delivery system by endocytosis results in a poorly defined endosomal trafficking process which, if not escaped, leads to degradation of the therapeutic DNA in lysosomes. For the small fraction of material that is able to escape this vesicular trafficking pathway, the cytosol provides additional physical and metabolic barriers to further trafficking to the nucleus. Finally, nuclear uptake has been demonstrated to be a significant barrier to gene delivery. In this review, we outline in greater detail the various processes involved in each step and describe various formulation variables that have been explored to overcome these delivery barriers to nonviral gene delivery.