Since the initial establishment of Robert Koch’s postulates in the nineteenth century, microbial protein toxins have been recognized as a major factor of bacterial and fungal virulence. An increasing number of proteins produced and secreted by various bacteria, yeasts and plants are extremely toxic and most of them developed remarkably "intelligent" strategies to enter, to penetrate and to finally kill a eukaryotic target cell by modifying or blocking essential cellular components. This book describes the strategies employed by protein toxins to render their pro- and eukaryotic producers a selective growth advantage over competitors. In providing an up-to-date overview on the mode of protein toxin actions, it accommodates biomedically and biologically relevant toxin model systems. As a result, it significantly broadens our perspective on biochemical architecture and molecular ploy behind the lethal principles of pro- and eukaryotic toxins.