Walter Fontana is Professor of Systems Biology at Harvard Medical School. His work on systems biology combines experimental and theoretical approaches, and they are oriented to the study of aging in C. elegans. He moved to Harvard Medical School in September 2004, attracted by a vision of systems biology that emphasized evolution and molecular physiology. A theoretician for 16 years, he were transformed by living for a while among molecular biologists and seeing the opportunities that quantitative thinking and technology bring to experimental biology. He started a group with a theoretical and an experimental component. The challenge of systems biology, in his own words, is not only experimental in kind: it also is the challenge of reasoning about facts that are rapidly evolving while remaining highly fragmented across research communities. Walter sees a fundamental role for models as reasoning instruments in biology. Models, not databases as we know them today, will become the main vehicles for the computer-assisted storage, communication, and retrieval of biological knowledge. Computer scientists and Walter have joined forces with several other researchers to design a computational environment that represents biological knowledge, as it pertains to signaling, in an editable and executable fashion. This instrument will lend itself to the collaborative construction and critique of models.