This is my own version od a lecture with a similar title of M. Dafermos, delivered at Stony Brook. (September 20010) .
This is a special lecture in honour of F. John's 100 anniversary. (November 20010) .
This is a Bourbaki seminar on the work of Dafermos-Rodnianski. (November 2009) .
This is a general lecture delivered at the international conference `` Beyond Einstein'' in Mainz, September 2008.
The set of lectures delivered at the IPAM workshop
Oscillatory Integrals and PDE' March 19-25 2001, UCLA.
This is another philosophical lecture delivered at
AMS Millenium Conference in Los Angeles, August, 2000.
Geometric and Fourier Methods in Nonlinear Wave
in Tel-Aviv, Aug, 1999).
This is a philosophical essay reflecting my
views about PDE's. Published in the proceedings of the conference ``
in Mathematics'' Tel Aviv 1999.