Abstract: 3'-deoxy-3'-¹⁸F-fluorothymidine (¹⁸F-FLT)is a promising tracer derived from thymidine. Accumulation of ¹⁸F-FLT is dependent on cellular thymidine kinase-1 (TK-1)activity. This enzyme is functional only in cells undergoing DNA replication during the S phase of the cell cycle.¹⁸F-fluorothymidine PET thus be able to image of cell proliferation in vivo for diagnosis, staging and monitoring of treatment re-sponse. Its advancements in basic and clinical studies will be reviewed in this article.