What do astronauts dream about when they are in space ? Always about Earth. And, once they have come back to Earth, what do they dream about ? Practically never about being in space... Similarly, Tama, a Bassari confronted to a metropolis like Paris, opens every night a suitcase containig African memories which feed his dreams.The accounts of the memories of dreams are difficult to interpret, because they are situated at the fronteer between individual experience and the dreamer's culture. At the conclusion of this essay which puts into question everything psychoanalysts have said about the contents and mostly the meaning of dreams, Michel Jouvet and Monique Gessain show that the memories of dreams, collected in "dream librairies", constitute a material which should not be neglected to better understand men and societies.Michel Jouvet, a member of the Académies des Sciences, discovered the mechanism of "paradoxal sleep". He is currently working in a sleep clinic where he studies the problems related to insomnia. He published two books at Ed. Odile Jacob, Le château des songes and Le sommeil et le rêve, which have been sold respectively in six and nine countries.