Joseph E. Alouf "The empirical basis of objective science has thus nothing ''abso lute'' about it. Science does not rest upon solid bedrock. The bold structure of its theories rises, as it were, above swamp. It is like a building erected on piles. The piles are driven down from above into the swamp, but not down to any natural or ''given'' base; and if we stop driving the piles deeper, it is not because we have reached firm ground. We simply stop when we are satisfied that the piles are firm enough to carry the structure, at least for the time being. " -Karl Popper (The Logic of Scientific Discovery) hen I was invited to write this introductory chapter I was hon W ored and pleased to have the opportunity to briefly highlight some important issues relevant to structural aspects of bacterial protein tox ins. The present book attempts for the first time to provide into one volume a series of chapters prepared by invited experts on the struc ture of these fascinating macromolecules. A particular emphasis is placed on the molecular features of these highly active biomolecules in rela tion to their biological properties and mechanisms of action. The researchers in bacterial protein toxinology of my generation have had the privilege of witnessing over 40 years the tremen dous development of this field of biological sciences, particularly in the past decade.