Gemcitabine was synthesised in the 1980s at Lilly Research Laboratories (Eli Lilly and Co., Indianapolis, IN). It was initially developed as an antiviral agent, but it had an extremely narrow therapeutic index due to its cytotoxicity against the parental cell lines. As it presented wide activity against various solid tumors it was then evaluated as an anticancer drug. This pyrimidine antimetabolite was firstly approved in 1996 by the Food and Drug Administration (FDA) for the treatment of pancreatic cancer. It is currently used for the treatment of many other malignancies such as lung cancer, breast cancer, lymphoma, bladder cancer, mesothelioma and ovarian cancer. This book discusses the molecular pharmacology, and the pharmacogenetics of gemcitabine. It also discusses the antineoplastic mechanism of gemcitabine on tumour cells; the systemic adverse effects; and provides detail on the clinical uses gemcitabine has on breast cancer.