Groups and clusters of galaxies are presently important objects of study, because they raise fundamental problems concerning structuring processes in the Universe. They comprise stellar matter, gas and unseen matter. The physical processes accounting for the evolution and interactions of these various components in the evolving Universe are complex.

From an observational point of view, the recent arrival of new and powerful instruments at various wavelengths, coupled with the ever increasing computing power, make the deepening of theoretical concepts and the improvement of numerical simulations timely. It is most likely that in the coming decade significant - if not final - answers will be given.

This meeting aimed at summing up the situation at the simultaneous beginning of a century, millenium and gold age of observational research.

Florence Durret, Daniel Gerbal

Welcome address

I am very glad to welcome you at IAP for our annual conference. After the successful conference on Large surveys last year,we enter today the topic of clusters of galaxies. It is a timely topic because we just have several new and outstanding instruments to probe these peculiar objects in the deep universe in many wavelengths or with a higher spatial resolution.

Andrea Biviano's historical talk and Simon White's talk will be the starting point of a lively conference, with stimulating discussions and possibly emerging collaborations between you.

Let me just say now a few words about our institute and its immediate future. IAP was one of the first CNRS laboratories, built in 1937. It strongly contributed to the development of astrophysics in France. Our ambition was to belong to the international network of the most dynamic astrophysical laboratories.

Our ministerial authorities are convinced that we shall maintain in astronomy such a pole of excellencies in Paris. They gave the kick off signal to engage a new development plan called IAP 2000+. In January 2001, we will be associated with the famous Pierre and Marie Curie University that will contribute to this effort hand in hand with the French CNRS.

We look forward to a rapid upgrading of the IAP building with new meeting rooms, teaching rooms, cafeteria, etc. plus 24 new offices for scientists. An architectural cabinet is presently working on a new design for IAP. I cross my fingers for a continuation of this funding until the end of this project.

This action is indeed taken to promote a new scientific strategy:

- To improve the field of theoretical astrophysics with the arrival of young theoreticians. In September, scientists working on physical cosmology, on the theory of stellar system formation and on the physics of compact objects will join IAP.

- Second but not least, to increase our visitor programme with more students and post docs coming from all over the world. We will advertise post-doc positions for the best students trained abroad during their Ph.D. thesis. It seems that concerning football our national team has never been so good since our guys mostly play in the best foreign clubs. We want to follow this example for intellectually competitive games.

I hope that in the next decade, IAP will be already experimenting a new foundation, but still with the former spirit of continuous evolution for even more creativity. The institute is small but wants to conjugate the cultural appeal of our old historical capital with the strong attractivity of a leading laboratory. We currently need your visit and help to succeed. Bring us your ideas and remarks. We need them to progress because we can learn a lot from all of you

I wish you a successful conference and from the depth of my heart, I thank you for being here.

Bernard Fort,

Directeur de l'Institut d'Astrophysique

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