Introduction : The School of Physics in Les Houches played the role of a trigger in the scientic life of Pierre-Gilles de Gennes. We know that from a recollections of Philippe Nozières entitled "Pierre-Gilles de Gennes, années de jeunesse". Indeed, in 1953, de Gennes and Nozières were in Les Houches as students (see Fig. 1) attending lectures, among others, of Rudolf Peierls. The encounter between the impressive master, one of the last encyclopedists and young students is related in lovely terms both by Nozieres as well as by Peierls him-self in his autobiography "Bird of passage: Recollections of a Physicist". Twenty years later, de Gennes came to Les Houches as professor and gave a lecture on hydrodynamics of nematic liquid crystals during the session "Molecular Fluids". The corresponding lecture notes, entitled "Nematodynamics" have been published in 1976. I chose them (among other 56 papers devoted to liquid crystals from more than 500 papers of de Gennes' Opera Omnia) to be reproduced and commented here