Most chemotherapeutic agents have a small therapeutic window and cause severe side effects. Prodrug therapy can increase the therapeutic window of cytotoxic agents. Linkage of a peptide specifier, which is a substrate for a tumor-specific enzyme, to a cytotoxic drug may yield a non-toxic and selectively activatible prodrug. A number of releasable linker technologies have been developed, which can be used to connect a specifier to a parent drug. One linker type concerns elongated spacers that position the specifier at sufficient distance from the parent drug. Both in vitro and in vivo, the plasmin-activated doxorubicin prodrug ST-9905, containing one of the novel linkers, has been demonstrated to possess superior drug release kinetics and antitumor efficacy when compared at equimolar and equitoxic dose with both Dox and ST-9802, which contains a conventional linker, without showing discernible side effects. Besides novel elongated linkers, other newly developed linker technol. will be presented.