In this introductory talk, I shall try to highlight a number of issues which will be addressed during the conference. To reduce the risk of trespassing on the territory of later speakers, I shall limit myself to general and tentative remarks. I shall try to focus on what we can learn about the origin of clusters and superclusters, emphasizing that almost all the observations to be discussed at this meeting are relevant to this issue. There are many classes of astronomical objects whose properties can be studied and understood in detail even in ignorance of their origins. To take an extreme example, our understanding of solar physics, and the general structure of the Sun at the present day, is in no way impeded by uncertainties about how it formed, or about star formation in general. This is, at least in part, because the characteristic timescales in the Sun are very short compared to its age: the Sun's dynamical timescale is less than an hour, so the period of solar oscillations is shorter by a factor of around 1014 than the Sun's total age. All dynamical memory of how the Sun formed was therefore erased long ago, and is irrelevant to our attempts to understand its present structure. In contrast, the dynamical timescales for clusters and superclusters are not much shorter than the age of the universe. These systems therefore retain an imprint of how they formed. Moreover, they are so bright that we can observe them out to high redshifts, and therefore at earlier cosmic epochs when they were less evolved. Clusters and superclusters tell us about structures in the early universe, and the subject of our conference impinges directly on cosmology.
Our knowledge of clusters and superclusters has expanded greatly within the last decade. But it is salutary to recall that the subject has a rather longer history. Indeed, the classic work of Shapley and Ames in the 1930s had already delineated many of the features of the largescale galactic distribution which exercise us so much today. From their studies of the distribution of galaxies brighter than 13th magnitude, Shapley and Ames were already able, in 1938, to delineate the Virgo cluster, several concentrations of clusters at greater distances, and draw attention to the asymmetry between the northern and southern galactic hemispheres. Further landmarks in the subject are associated with the names of Abell, de Vaucouleurs, Oort, Einasto, and many others. It is on these pioneering foundations that later work, including contributions to be reported at this meeting, has been built.