The appearance and activation of macrophages are thought to be rapid events in the development of many pathological lesions, including malignant tumors, atherosclerotic plaques, and arthritic joints. This has prompted recent attempts to use macrophages as novel cellular vehicles for gene therapy, in which macrophages are genetically modified ex vivo and then reintroduced into the body with the hope that a proportion will then home to the diseased site. Here, we critically review the efficacy of various gene transfer methods (viral, bacterial, protozoan, and various chemical and physical methods) in transfecting macrophages in vitro, and the results obtained when transfected macrophages are used as gene delivery vehicles. Finally, we discuss the use of various viral and nonviral methods to transfer genes to macrophages in vivo. As will be seen, definitive evidence for the use of macrophages as gene transfer vehicles has yet to be provided and awaits detailed trafficking studies in vivo. Moreover, although methods for transfecting macrophages have improved considerably in efficiency in recent years, targeting of gene transfer specifically to macrophages in vivo remains a problem. However, possible solutions to this include placing transgenes under the control of macrophage-specific promoters to limit expression to macrophages or stably transfecting CD34(+) precursors of monocytes/macrophages and then differentiating these cells into monocytes/macrophages ex vivo. The latter approach could conceivably lead to the bone marrow precursor cells of patients with inherited genetic disorders being permanently fortified or even replaced with genetically modified cells.