In recent decades the biological sciences have rapidly moved from being qualitative to being quantitative.  In this talk, we will report some recent work that applies concepts and tools from nonlinear science to address biological problems. We examine two examples: the cell cycle control networks of brewer’s yeast (Saccharomyces cerevisiae, see picture), and the p53 signal transduction pathway of a mammalian cell. This work demonstrates the value of nonlinear science in biological studies, but we will show that additional mathematical methods need to be developed for applications in systems biology.