This book offers a thorough exploration of the observation of Earth's water–energy cycle, a foundational aspect of climate science. It features a collection of up-to-date articles that provide insights into current estimates of the Earth’s water and energy fluxes, based on satellite observations, atmosphere–ocean reanalyses, and global coupled atmosphere–ocean model simulations. The book deepens our understanding of the role of clouds in the energy cycle, focusing particularly on tropical clouds and their responses to surface warming patterns, which influence global warming. It also examines the connection between tropical clouds and deep convection in the tropics. Furthermore, the volume identifies key priorities for developing an integrated and optimized observation system for Earth's water–energy cycle. It highlights the challenges currently faced in monitoring and modeling this cycle to improve predictions on annual to multi-decadal timescales. Reprinted from Surveys in Geophysics, Special Issue: Earth's Changing Water and Energy Cycle, Vol. 45, 2024.