This book contains a series of lectures given at the NATO Advanced Study Institute (ASI) "Structure Formation in the Universe", held at the Isaac Newton Institute in Cambridge in August, 1999. The ASI was held at a critical juncture in the development of physical cosmology, when a flood of new data concerning the large scale structure of the Universe was just be­ coming available. There was an air of excitement and anticipation: would the standard theories fit the data, or would new ideas and models be required? Cosmology has long been a field of common interest between East and West, with many seminal contributions made by scientists working in the former Soviet Union and Eastern bloc. A major aim of the ASI was to bring together scientists from across the world to discuss exciting recent developments and strengthen links. However, a few months before the meeting it appeared that it might have to be cancelled. The war in the former Yugoslavia escalated and NATO began a protracted bombing cam­ paign against targets in Kosovo and Serbia. Many scientists felt uneasy about participating in a NATO-funded meeting in this situation. After a great deal of discussion, it was agreed that the developing East­ West conflict only heightened the need for further communication and that the school should go ahead as planned, but with a special session devoted to discussion of the legitimacy of NATO's actions.