The intellectual landscape of Biology is in a state of upheaval: Our field has focused for so long on the "parts lists" of life--systematically enumerating genomes, proteomes, phenomes, etc.--that now, as many of these lists attain a very real measure of completeness, we are being challenged to re-evaluate the very nature of what we do. Systems Biology , at least as we use the term here, refers to a way of approaching life science that, if not entirely new, has only recently become of widespread utility.
To the Systems Biologist, the living world is built of interacting sets of complex systems, crafted by natural selection to carry out defined tasks. Understanding such a world requires addressing questions with an engineering flavor, such as "Why is a system designed the way it is?" "How does it function robustly?" and "What can the system be altered to do?" To tackle such questions, one needs to know not just mechanisms ("how does it work?") but also purposes ("what is its job?" "why was it advantageous to build it that way?"). Yet relationships between how complex systems are built and what they were designed to do are usually far from intuitive; considerable computational effort and mathematical skill are often needed to make the necessary connections. Such a strong dependence on ideas and approaches outside the discipline of Biology explains why Systems Biology, in our view, is not just a trend within biology, but a truly hybrid discipline, an amalgam of molecular biology, evolutionary biology, engineering, mathematics and computer science.