The complexity of ecosystems forms perhaps the greatest challenge for natural science. Even the first step to comprehensive analysis, namely a survey of the participating species, often forms a major obstacle. This makes it understandable that ecologists try to abstract general principles from the interrelationships of the multitude of species for use in their efforts to investigate ecosystem dynamics. Such 'unifying concepts' were the main theme of the 'First International Congress of Ecology' organized in The Hague in September 1974 by the International Association of Ecology (INTECOL), under the auspices of the Division of Environmental Biology of the International Union of Biological Sciences (IUBS). This book contains the papers presented at the plenary sessions of the Congress and a summary of the discussions engendered by them. At the Congress over 800 ecologists from many countries, representing diverse disciplines such as limnology, botany, zoology, microbiology, agriculture, met together for a week. The study of ecosystem dynamics depends on mutual understanding and close cooperation, and to stimulate an integrated approach a number of main speakers were invited to contribute papers on notions such as energy flow, productivity, diversity, stability and maturity from different points of view. These invited papers were presented at the morning plenary sessions, followed by discussions.