The Earth’s climate is changing. Multiple lines of evidence show changes in our weather, oceans, ecosystems, and seasonal events. This technical report summarizes the current state of the science on observed changes related to seasonality in the United States and discusses how climate change affects the timing and nature of seasonal events. The report uses several key indicators sensitive to and related to seasonality as a framework for understanding the implications of a changing climate over time. The indicators are based on long-term observations and reveal the many dimensions of seasonal events, including critical connections between physical changes and biological responses. The report provides examples of how changes in seasonality affect ecological and human systems, as well as our everyday lives.