The text is aimed at a broad audience of students and other individuals interested in furthering their understanding of how cells regulate and coordinate their core activities. Malfunction in the control layer is responsible for a host of human disorders ranging from neurological disorders to cancers. Most drugs target components in the control layer, and difficulties in drug design are intimately related to the architecture of the control layer. This book will be useful to students and individuals in medicine and pharmacology interested in broadening their understanding of how the control layer works. To further that goal, there are chapters on cancers and apoptosis, and on bacteria and viruses. In those chapters not specifically devoted to pathogens, connections between diseases, drugs and signaling are made. The audience for this book includes students in chemistry, physics and computer science who intend to work in biological and medical physics, and bioinformatics and systems biology. The first five chapters of the book are mainly background and review chapters. Signaling in the immune, endocrine (hormonal) and nervous systems is covered along with cancer, apoptosis and gene regulation.