Bacterial toxins that act inside cells interact very specifically with key components of the cell and some even manipulate the cell in subtle ways for their own purposes. These potent toxins, described in this book, will be of interest to both microbiologists and cell biologists. Some of these toxins are conventional multidomain toxins that are self-programmed to enter cells. Others are delivered by type III mechanisms, often as a package of potent molecules. The molecular targets for all these toxins mediate signal transduction and the cell cycle to regulate the crucial processes of cell growth, cell division and differentiation. Thus these potent toxins are not only responsible for disease, but also provide a powerful set of tools with which to interrogate the biology of the cell. In addition such toxins may act directly to promote carcinogenesis and hence their study is also of interest in a wider context.