Sixth Mississippi State Conference on Differential Equations and Computational Simulations. Electronic Journal of Differential Equations, Conference 15 (2007), pp. 221-228. Title: Louis Nirenberg and Klaus Schmitt: The joy of differential equations Author: Jean Mawhin (Univ. Catholique de Louvain, Belgium) Abstract: A tribute to Louis Nirenberg for his 80th birthday anniversary and to Klaus Schmitt for his 65th birthday anniversary. Published February 28, 2007. Math Subject Classifications: 01A70, 01A60,34-03, 35-03. Key Words: Nirenberg; Schmitt; differential equations.