This week marks the launch of a new, improved version of the AAAS journal Science Signaling , which will now contain original research. Professor Michael Yaffe, its new chief scientific editor, has clearly articulated the ambitious goals for his journal.Highly elaborate signaling mechanisms are essential for controlling the behavior of each cell in a multicellular organism--allowing each of the many billions of cells in our bodies to decide whether it will grow and proliferate, remain quiescent, kill itself, or change its behavior according to signals received from neighboring cells. Understanding how this complex system works represents a major challenge. Unraveling its many mysteries will require a great deal of ingenuity--and the collaboration of biologists, chemists, physicists, engineers, computer scientists, and mathematicians. We are confident that Science Signaling will set the highest standard for research in this important field, and that, through its Perspectives and Review articles, it will help to guide future researchers along highly productive paths.