Stable isotopes of oxygen and hydrogen in water (“water isotopes”) are powerful tools for understanding how Earth’s climate system works, both today and in the past. In this talk, I introduce how variations in water isotopes record information about rainfall, evaporation, and ocean–atmosphere interactions in the tropical Pacific. I will highlight two areas of our current research: measurements of modern precipitation and seawater isotopes in the Galápagos and Palau, and what they reveal about present-day hydroclimate processes. Next, I will present a new paleoclimate reconstruction from coral atoll lagoon sediments on Kiritimati (Republic of Kiribati). Oxygen isotope values in fossil ostracode valves from these cores shed light on past climate variability in this sensitive region.