Historical Comments on Monge's Ellipsoid and the Configuration of Lines of Curvature on Surfaces Immersed in {mathbb R}³

This is an essay on the historical landmarks leading to the study of principal configurations on surfaces, their structural stability and further generalizations. Here it is pointed out that in the work of Monge, 1796, are found elements of the qualitative theory of differential equations, founded by Poincar\'e in 1881. Two open problems are proposed.