Tilli Tansy is Reader in Modern Medical Science, UCL. Her published original contributions are many in number, high in quality and far ranging in their impact. Without her work, our understanding of the development of much of modern biomedical science, particularly physiology, pharmacology and neuroscience in general, and the pharmaceutical industry in particular, would be much weaker. The historical assessment and analysis of the role of animal experimentation in recent biomedical science also runs through much of her research. Of comparable importance is her success in stimulating new methods of research as leader of the History of Modern Medical Science group at the Wellcome Trust Centre at University College London. Significantly, she initiated an entirely new methodology, that of the Witness Seminar, to record aspects of modern biomedicine that would otherwise be lost. This technique has captured essential source material in more than forty wide-ranging areas of biomedical research and is now mirrored around the world. Without her unique methods we would have lost, and would continue to lose, a wealth of historical material.