Despite advances in cancer biology, chemotherapy remains the backbone of treatment approaches for many patients with metastatic breast cancer (MBC). Halichondrins, derived from marine sponges, have significant potential as potent antimicrotubule agents. Eribulin, with proven preclinical activity, is a synthetic halichondrin analog with novel actions on tubulin including suppression of microtubule polymerization. Phase I and II studies in MBC identified neutropenia as the dose-limiting toxicity and a maximum tolerated dose of 1.4 mg/m2 on days 1 and 8 of a 21-day cycle. An encouraging response rate of 11.5% in refractory MBC led to the launch of the phase III Eisai Metastatic Breast Cancer Study Assessing Physician's Choice versus Eribulin trial, in which heavily pretreated patients with MBC were randomized 2 : 1 to intravenous eribulin or monotherapy of the investigator's choice. Recently, it was reported that this important study of 762 patients met its primary endpoint of overall survival: eribulin was associated with an improvement in median overall survival from 10.65 months to 13.12 months (hazard ratio 0.8; 95% confidence interval 0.66-0.99) and a response rate of 12.2%. In general, the side effect profile of eribulin seems to be acceptable, as although neutropenia occurred in 45% of the patients, febrile neutropenia was rare and the incidence of neuropathy was low. These findings show that eribulin is potentially a new active agent for MBC, although results of ongoing studies are awaited to confirm the reported benefit. Future studies will investigate the potential role of eribulin in other settings, including for early breast cancer, to ascertain how to optimally incorporate this new agent into current treatment paradigms.