This volume is aimed at mathematicians and advanced students interested in buildings, groups and their interplay. Its first part, which presupposes a good knowledge of ordinary buildings, looks at the theory of twin buildings. The second part provides solutions and the background to problems.

This text started out as a revised version of Buildings by the second-named author 53], but it has grown into a much more voluminous book. The earlier bookwasintendedtogiveashort, friendly, elementaryintroductiontothet- ory, accessibletoreaderswithaminimalbackground.Moreover, itapproached buildings from only one point of view, sometimes called the "old-fashioned" approach: A building is a simplicial complex with certain properties. The current book includes all the material of the earlier one, but we have added a lot. In particular, we have included the "modern" (or "W-metric") approach to...