This book had its origins in some conversations held several years ago. It was felt that a monograph on the subject of tissue specificity might have some merit and interest for a number of scientific workers in the relevant disciplines. Writing was begun slowly, with interruptions; more recently, there has been much more activity, and the relevant literature has been searched and cited up to early 1973. This book was never intended as an exhaustive coverage of all possible topics related to tissue specificity. The tissues and organs covered in six chapters constitute a selected group, representative of areas in which the author has worked and which are thus directly familiar to him, plus several other areas where illustrative and important discoveries have been made. There are other areas of equal importance and interest, but the book would have been much too large if I had tried to cover all of them. The author had several groups of readers in mind, including students of immunology and the related fields of immunochemistry, biochemistry, and immunopathology. Students of general biology could find it useful. For students of medicine and practicing physicians it may provide a useful review in a field that has many clinical correlations in human disease. The author assumes that the reader has a basic knowledge of immunological concepts and technics, but for some purposes elementary treatments are cited for background reference.