Four years ago we edited a volume of 36 papers entitled Molecular Approaches to Ecology and Evolution (Schierwater et ai. , 1994), in which we attempted to put to­ gether a diverse array of papers that demonstrated the impact that the technologi­ cal revolution ofmolecular biology has had on the field ofevolutionary biologyand ecology. The present volume borrows from that theme but attempts to focus more sharply on the impact that molecular biology has had on our understanding of dif­ ferent hierarchical levels important in evolutionary and ecological studies. Because DNA sequence variation is at the heart ofeverypaper in the present volume, we feel it necessary to examine how DNA has affected study at various levels of biological organization. The majority of the chapters in the present volume follow themes es­ tablished in the earlier volume; all chapters by authors in the previous volume are either fully updated or entirely new and expand into areas that we felt were impor­ tant for a more complete understanding of the impact of DNA technology on ecol­ ogy and evolution. The collection of papers in this volume cover a diverse array of ecological and evolutionary questions and demonstrates the breadth of coverage molecular tech­ nology has imparted on modern evolutionary biology. There are also a broad range of hierarchical questions approached by the 17 papers in this volume.