This book examines the role of photoperiod (day length) in timing seasonal adaptations in plants, invertebrates, and vertebrates, and is the first to present such a broad perspective on the subject in quite some time. The current literature is distinctly separated among researchers working with these different taxa, resulting in inefficiency and redundancies. The field is poised to make rapid progress in the understanding of seasonal clocks at all levels of analysis, and Photoperiodism brings together experts working in disparate areas to stimulate conversation among investigators from all related disciplines. At the end of the book, the three editors analyze common themes in photoperiod time measurement across taxa, as well as common and dissimilar approaches to the study of photoperiodism, and propose future directions in research on photoperiodic time measurement.