Imaging techniques are currently being developed to map the topography and level of gene expression following gene therapy. To date, two different imaging strategies have been investigated--using marker genes encoding either intracellular enzymes or cell-surface receptors. The first approach employs the ability of certain enzymes to modify imaging prodrugs, so that tissue accumulation of such drugs reflects the expression. The second approach utilises cell-surface expression of a ligand-binding receptor that can be detected using imaging tracers. In this review, we discuss nuclear-and magnetic-resonance-image techniques that have been developed to detect gene expression.