The chicken is a useful model for research into the basic features of immunology because its immune system functions in a way similar to that of human beings and because the chicken embryo is easily accessed experimentally. This book is a collection of comprehen­ sive articles updating chicken embryogenesis and immunology. It is now clear that it is possible to produce transgenic chickens by embryonic stem cell manipulation and transfer into the oocyte. Alternatively, the chick offers us the possibility of producing somatic chimeras by manipulating embryonic cells at the primitive streak or earlier stages. Working with later em­ bryos, we can learn the principles involved in the positioning of organ anlagen such as the limb bud. It is shown in this book that these structures are formed by appropriate cell and tissue arrangements driven by molecular factors. Work on the chicken has contributed substantially to our understanding of basic immunology, including the graft-vs. -host reaction, the clonal reactivity of immunocompetent cells, sepa­ ration of the T and B cell lineages, somatic diversification of the B cell repertoire by immunoglobulin (Ig) class switching, anti­ body diversification, Ig gene conversion, and the origin of the hematopoietic stem cell precursor of lymphoid and myeloid cells. Hematopoietic stem cells emerge successively from sev­ eral embryonic sites such as the yolk sac, the aorta, and the mesentery near the aorta.