Most galaxies in the Universe live in groups, experiencing a broad range of environments, and at the same time manifesting diverse properties. Comparative studies of galaxy groups and group members, and detailed studies of individual groups, promise an understanding of the evolution of the groups themselves as well as their stellar systems. Historically, the former approach was pursued for nearby groups, while the latter was restricted to the Local Group. The recent advance of technology, and in particular the availability of 8-10 meter class telescopes, allows both approaches to be combined. Furthermore, with the next generation of extremely large telescopes and space missions, it will be possible to study nearby groups at the same level of detail as is currently achievable for the Local Group.

To address the unanswered questions on the nature of galaxy groups, we will hold a conference aiming to:

• summarize the considerable progress that has been made since the last meetings dedicated to galaxy groups (The IAU Coll. 174 on Small Galaxy Groups, eds. Mauri J. Valtonen and Chris Flynn; ASPC 209; and Groups of Galaxies, ASPC 70, eds. Otto-G. Richter and Kirk Borne, in the years 2000, and 1995, respectively).
• formulate the new problems that can be tackled both with the new instrumentation available to the community, and with the ever advancing theoretical work.

As a guideline, we propose to discuss the following topics:

1. Search, definition, classification, and statistics of groups; comparison with clusters and field; surveys (imaging, spectroscopy) at all wavelengths: X-rays, optical, IR, etc.;
2. Evolution of galaxies in groups: DM, gas, dust; Stellar and galaxy populations in groups; mergers, starbursts, nuclear activity; galactic chemical evolution;
3. Evolution of groups as a whole: Dynamical evolution, chemical evolution of the intragroup medium; galactic luminosity function; Role of environment (density);
4. Groups in the context of cosmological structures: Comparison with clusters of galaxies and field population; Group environment; Distances to groups; Theoretical/numerical simulations of groups and the intra-cluster/intra-group medium; Sunyaev-Zel'dovich effect;
5. Extreme groups: densest, largest, closest, with most peculiar galaxy population, etc.;
6. The future:
   1. the fate of nearby groups; “fossil” groups;
   2. future avenues of research, both theoretical and observational (SIM, Gaia, OWL, GMT, JWST, ALMA, etc.

E-MAIL: groups2005@eso.org