The HSV1-tk gene has been explored as a reporter and/or suicide gene in molecular imaging of gene expression. Gene therapy with HSV1-tk and the use of this gene as a marker have been applied in patients with various forms of cancer. However, the conditions for clinical gene therapy protocols have yet to be optimized. A method to monitor the activity of HSV1-tk in vivo would be extremely useful to optimize clinical gene therapy protocols. Positron emission tomography (PET) with a suitable probe can offer information about both the extent of gene expression and its distribution, provided that an appropriate reporter gene is included in the therapeutic cassette. PET imaging provides higher resolution and sensitivity and allows noninvasive quantification of biological processes. Several radiolabeled pyrimidine (thymidine) and purine (acycloguanosine) derivatives have been developed as reporter probes for imaging of HSV1-TK enzyme activity with PET. In this review, information on radiolabeling and PET imaging of HSV1-tk gene expression with various nucleoside analogues is presented.