This monograph deals with the impact of classical genetics in immunology, providing examples of how large immunological questions were solved, and new fields opened to analysis through the study of phenotypes, either spontaneous or induced. As broad as biology has become, there are those who don’t fully understand what the genetic approach is, and how it differs fundamentally from most of the methods available to natural scientists. They may hold the opinion that genetics has run its course since Mendel read his paper on peas in 1865. “Why bother with classical genetics,” they may ask. “Won’t all genes be knocked out soon anyway?” Or they are intimidated by genetics, with its heavy reliance on model organisms that seem so alien. “What has C. elegans to do with me?” they may ask. “It doesn’t even have lymphocytes.” They may be unaware that the mouse is fast becoming as tractable a model organism as the fly, and that humans may not be too far behind.