In this collection of essays, some of the leading ecologists and philosophers discuss the foundations of ecology and evolutionary biology. While large­ scale philosophical convictions and attitudes often direct the theorist's line of concrete action in data collection and in theory information, the founda­ tional convictions typically remain tacit, and are seldom argued for. The present collection aims to remedy this situation. It brings together scholars representing different approaches in a joint effort to explicate and analyse some of the key issues underlying ecological theorizing, be they conceptual, epistemological or ontological. The bulk of the present collection is reprinted from Synthese 43 (1980). William C. Wimsatt's paper 'Reductionistic Research Strategies and Their Biases in the Units of Selection Controversy' is in turn reprinted from T. Nickles (ed.) Scientific Discovery: Case Studies (D. Reidel, 1980). It appears here with the kind permission of Prof. Nickles. The publisher's consent for the reprints has been in each case automatic. The essays of Y rjo Haila and Olli Jarvinen, and of Leigh M. Van Valen appear here for the first time. In bringing the present collection together, as well as in editing the Synthese symposium on which it is based, I have greatly benefited from the suggestions of Professors Marjorie Grene, Olli J iirvinen and Daniel Simberloff. In addition to them, I wish to thank all the contributors for their interest in this project.