Our understanding of ecological responses to climate change is largely limited by a lack of long-term and spatially extensive data on appropriate ecological variables. The amount of data collected by scientists is particularly limited, in part because there has historically been little funding for long-term research of this sort. Data collected by citizen scientists have filled much of this data gap and have led to many important insights into our understanding of how plants, animals, and ecosystems are responding to climate change. For example, volunteers’ observations of flowering, leaf out, and bird migrations have provided key examples of climate-driven shifts in phenology. Records of plant, bird, and butterfly sightings have similarly shown shifts in species ranges. In fact, much of our most important, spatially extensive, ecological data have been collected by citizen scientists participating in programs such as the North American Breeding Bird Survey, Christmas Bird Counts, International Phenology Gardens, eBird, bird, butterfly, and plant atlases, and predecessors to the USA National Phenology Network (e.g., lilac and honeysuckle observation networks). From these datasets and others like them, we are beginning to understand important aspects of interspecific and spatial variation in species responses to climate change. Scientists sometimes question the quality of data collected by citizen scientists, which has perhaps limited the number of studies that utilize these data. However, recent studies have shown that citizen science data are often comparable to professionally collected data and can be relied upon to contribute to science and management decisions. Here we highlight key insights into ecological responses to climate change derived from citizen science data, programs that have yielded these data, and new initiatives that plan to expand, standardize, and utilize observations made by citizen scientists.