This volume is a collection of fourteen papers, written by well-known authors, on aspects of applied mathematics, fluid dynamics, combustion, kinetic theory, condensed matter physics, computational neuroscience, biophysics and closely related areas. There are two uniting themes. First, the papers celebrate the long and durable contributions of Professor Lawrence Sirovich on the occasion of his turning seventy. Second, the threads of nonlinearity weave through all the problems discussed. The papers combine original research with expository style and make a fascinating reading for a diverse readership in applied mathematics and science.