A number of years ago Prof. J. Peiffer (Tiibingen/FRG) asked whether cerebros- nal fluid cells originate in blood cells. With this question, the first seeds were sown for the research which resulted in this book. The original question was eventually ex panded to whether extravascular hematogenous cells are present in the nervous sys tem. In the course of my research this question became more and more concentrated on the demonstration of monocyte derivatives (i.e., mononuclear phagocytes) within the nervous system. While these questions are certainly not new, they regularly crop up when micro...