How new species emerge has fascinated biologists since Darwin. Barriers to gene flow, such as sexual isolation, habitat selection, or postzygotic inviability, play critical roles. These lead to reproductive isolation and are crucial to the formation of new species. Written and edited by experts in the field, this collection from Cold Spring Harbor Perspectives in Biology reviews progress in our understanding of the speciation process. Recent advances have shed new light on how reproductive barriers are established, how these barriers to gene flow are coupled to each other, and the underlying genetic mechanisms. The contributors cover innovations in genetics, genomics, theoretical frameworks, and comparative techniques that have broadened the perspective of researchers in the field. They review the contributions of pre- and postzygotic isolating barriers, hybridization, chromosomal architecture, biotic and abiotic factors to speciation, particularly in cases of adaptive radiations. Speciation research has often become entangled with cultural and political views. This problematic history is also addressed, as is the impact of human activity on speciation. The volume will therefore be of interest not only to geneticists and evolutionary biologists but ecologists and other scientists interested in understanding how speciation occurs and its consequences. Catherine Peichel is Head of the Division Evolutionary Ecology at University of Bern Institute of Ecology and Evolution Daniel Bolnick is a Professor in the Department of Ecology and Evolutionary Biology at the University of Connecticut Rebecca Safran is Professor of Ecology and Evolutionary Biology at the University of Colorado Boulder Department of Environmental Studies Åke Brännström is Professor in the Department of Mathematics and Mathematical Statistics at Umea University