The idea of a Colloquium on “Cosmology: Facts and Problems” was mooted when one of us (JVN) was to visit Collège de France as Professor (Chaire Internationale) during 2003–04. Both of us felt that the subject of cosmology has seen considerable advancement on both observational and theoretical fronts but that there are many issues of observational nature that will remain to be understood. With this point of view the Colloquium was arranged during June 8–11, 2004, at Collège de France.

The Colloquium attracted leading workers in the field. They could be divided into three categories: 1. Observers 2. Theoreticians who liked to explain all the observed data in terms of the standard big-bang paradigm 3. Theoreticians who felt that there were some observations that did not allow a standard interpretation. Sometimes the observers also fell under categories 2 and 3. We were happy that the Colloquium attracted good participation from several countries and there was amiable and frank discussion on various issues. We had allowed plenty of time for discussion after each presentation including a panel discussion at the end. The proceedings presented here reflect this openness of the debate. Several participants who had not given a formal presentation also took part in the discussion.

We would like to express our grateful thanks to all those who helped us in various ways towards making this Colloquium such a success. In particular, we would like to thank Professor Jacques Glowinski, Administrateur du Collège de France, for his kindness in hosting the Colloquium at the Collège de France.