The discovery of specifically acquired immunity which followed the major contributions of Louis Pasteur completely over-shadowed the first studies of the host's natural resistance. Later, the exquisite sensitivity and precision of antigen-antibody reactions made the study of immunochemistry much more attractive than the rather primitive and ambiguous field of non-specific immunity. Neverthe less, during the last three decades, a considerable body of informa tion was developed and also means by which natural resistance could be enhanced or depressed by exogenous agents such as lipopolysaccharides or BCG. An important advance was the chemical recognition of the biologically active components of these agents which in turn allowed the synthesis or" analogues. More recently, endogenous host products which can play a role in nonspecific immunity, such as thymic hormones, have also been identified, produced and used both experimentally and clinically. It therefore seemed worthwile to Drs. Miescher and Mueller-Eberhard to devote two volumes of Seminars in Immunopathology to the topic of Immunostimuhltion. Because of the good response obtai ned from readers, Springer Verlag decided to issue a hard cover book and asked their guest editor to make a preface. Prefaces, although they are found in the opening pages, are always written after the first issue has been completed."