Periodic plant and animal cycles driven by seasonal variations in climate (i.e., phenology) are critical to the function of ecosystems and human society. They regulate the dynamics of ecosystem processes, determine land surface properties, control biosphere-atmosphere interactions, and affect food production, health, conservation, and recreation. Phenological events also comprise some of the most readily and widely recognized ecological processes—first leaves in the spring, coloration of fall foliage, migrations of many birds, insects, and mammals, and flowering of plants. Recent meta-analyses suggest that phenology is among the most sensitive indicators of the biological response to environmental variation and climate change. Thus, phenology is increasingly recognized as an invaluable tool for predicting future ecological change and for educating the public on the effects of global changes on natural systems. The list of groups potentially interested in applying phenological data and models to contemporary problems is extensive—research scientists, educators and outreach specialists, conservation biologists, natural resource managers, and professionals in the fields of agriculture, tourism and recreation, and human health. These groups have recently come together to form the USA National Phenology Network (www.usanpn.org), a program to monitor plant, animal and landscape phenology at a national to continental scale and to catalog long-term phenological datasets. Here, we provide a diverse set of posters to show the history and current state of our understanding of phenology, highlight some case studies of the applications of phenological data and models, describe growing efforts to monitor phenology in a systematic manner, and outline how phenology can be used to engage the public in ecology and climate change science.