Chapter 2 explores the question: “What are the parts of the climate system and how do they interact”? This exploration begins by introducing the five interconnected subsystems, or spheres, of the climate system (the hydrosphere, cryosphere, biosphere, atmosphere, and lithosphere). More detailed discussions explore the compositions, structures, and transfer/circulation processes that operate within each sphere, and that transfer and store energy, water, and carbon within and between those systems. These processes and transfers interact with external climate forcings over different timescales and spatial scales to control climate. Understanding these spheres, processes, transfers, and forcings lays the foundation for interpretations of past climates and predictions of future climates, as is discussed in later chapters of this book.