GREAT PROBLEMS IN NONLINEAR EVOLUTION EQUATIONS <>
This is another philosophical lecture delivered at the AMS Millenium Conference in Los Angeles, August, 2000.

PDE AS A UNIFIED SUBJECT <>
This is a philosophical essay reflecting my personal views about PDE's. To appear in the proceedings of the conference `` Visions in Mathematics'' Tel Aviv 1999.

A COMMUTING VECTORFIELD APPROACH TO STRICHARTZ TYPE INEQUALITIES AND APPLICATIONS TO QUASILINEAR WAVE EQUATIONS
This paper marks my whole- hearted return to quasilinear wave equations, after a detour of more than ten years. I show that one can recover some of the recent results of Chemin-Bahouri and Tataru by reducing the crucial dispersive inequality( the key ingredient in the Strichartz type estimates) to L^2--L^\infty decay estimates based on energy estimates and an adaptation of the commuting vectorfields method.

Some General Remarks on Nonlinear PDEs
Geometric and Fourier Methods in Nonlinear Wave Equations)
(Lecture in Tel-Aviv, Aug, 1999)

Bilinear Estimates and Applications to Nonlinear Wave Equations
This is a survey paper in collaboration with S. Selberg concerning  optimal well posedness
results for nonlinear wave equations such as Wave Maps, Maxwell-Klein-Gordon and
Yang Mills. The survey provides a unified treatment  and simplified proofs    for some old results of Klainerman-Machedon,  Klainerman-Selberg, Klainerman-Tataru.