Why has the Earth, alone of all the planets, been able to support life for at least 3.7 Billion years? What factors have regulated and maintained the Earth’s climate near habitable conditions for so long? How do environmental changes of the past inform our understanding of the world we live in today? And how do we know so much about Earth’s past? We will examine the principal sources that provide energy to the Earth’s surface and modify its composition over time: solar input, carbon and tectonic cycles, and biological processes. Our job as Earth scientists is to identify the processes that shape the Earth’s surface environment, assess the characteristic timescales on which they operate, envision how the strengths of different processes could have changed over time to yield different Earth states, and study the Earth’s past using a variety of geologic tools.