This volume contains the proceedings of a conference held in celebration of the seventieth birthdays of Peter Lax and Louis Nirenberg at Villa la Pietra in Florence (Italy). Speakers from around the world gave talks on subjects related to the mathematical areas in which Lax and Nirenberg worked: analysis, partial differential equations, applied mathematics and scientific computing. The two men played seminal roles in these areas and had significant influence on the development of many other mathematicians. This volume gives testament to the major role played by Lax and Nirenberg in the development of mathematical analysis.