This eighth edition of Cellular and Molecular Immunology includes substantial additions and revisions, which we made to keep the textbook current with scientific advances and, at the same time, maintain the clear and readable style that has been typical of past editions. Whenever we have added new information, we have focused primarily on important concepts and have not increased the length of the book. We have also rewritten many sections for increased clarity, accuracy, and completeness. Among the major changes is a reorganization of the chapters dealing with T lymphocyte responses in order to more clearly describe early T cell activation events, differentiation and functions of subsets of CD4+ helper T cells, and functions of CD8+ cytotoxic T cells. In addition, the entire book has been updated to include important recent advances in immunology. Some of the topics that have been significantly revised are innate lymphoid cells, the developmental pathways of macrophages and dendritic cells, and immune checkpoints in tumor immunity. It is remarkable and fascinating to us that new principles continue to emerge from analyses of the complex systems that underlie immune responses. Perhaps one of the most satisfying developments for students of human disease is that basic principles of immunology are now laying the foundation for rational development of new immunologic therapies. Throughout the book, we have tried to emphasize these new therapeutics and the fundamental concepts on which they are based. We have also continued to improve our illustration program. New figures have been added, and previously used figures have been reviewed and often changed for accuracy and clarity. We have kept design features such as the use of bold italic text to highlight “take-home messages” to make the book easy to read. The lists of suggested readings continue to emphasize recent review articles that provide in-depth coverage of particular topics for the interested reader. We have divided the lists into sections based on themes to help readers find the most useful articles for their needs.