Epothilones A and B are naturally occurring microtubule-stabilizers, which inhibit the growth of human cancer cells in vitro at nM or even sub-nM concentrations. In contrast to paclitaxel (Taxol®) epothilones are also active against different types of multidrug-resistant cancer cell lines in vitro and against multidrug-resistant tumors in vivo (epothilone B). Their attractive preclinical profile has made epothilones important lead structures in the search for improved cytotoxic anticancer drugs and epothilone B is currently undergoing phase II clinical trials. Numerous synthetic and semi-synthetic analogs have been prepared since the absolute stereochemistry of epothilone B was first disclosed in mid-1996 and their in vitro biological activity has been determined. Apart from generating a wealth of SAR information, these efforts have led to the identification of at least four compounds (in addition to epothilone B), which are currently at various stages of clinical evaluation in humans. This review is first intended to provide a summary of the basic features of the in vitro biological profile of epothilone B, with particular emphasis on recent developments in this area. A second part will outline the most relevant aspects of the epothilone SAR with regard to effects on tubulin polymerization, in vitro antiproliferative activity, and in vivo antitumor activity. This will include a brief discussion of research directed at the determination of the bioactive conformation of epothilones. In a final section, the preclinical profile of those epothilone analogs currently in clinical development will be discussed in greater detail.