Earth's climate has never been static. Over hundreds of millions of years, our planet has experienced ice ages, greenhouse climates, shifting sea levels, mass extinctions, changing ocean circulation, and dramatic reorganizations of ecosystems. These ancient environments are recorded in rocks, fossils, sediments, ice cores, soils, isotopes, and the changing distribution of plants and animals. By learning how geologists and climate scientists interpret these records, we can reconstruct past worlds and understand the physical processes that shaped them. This presentation will examine how Earth's climate has changed through time, what drove those changes, and how scientists distinguish natural climate variability from externally forced change. We will consider the roles of plate tectonics, volcanic activity, atmospheric greenhouse gases, orbital cycles, ocean circulation, ice sheets, weathering, and biological evolution. Particular attention will be given to what past climates reveal about climate sensitivity, feedback mechanisms, tipping points, and the pace of environmental change. The central theme is that the past is not merely a record of what once happened; it is a natural laboratory for understanding how the Earth system works. Ancient climate change does not provide a simple one-to-one prediction of the future, but it does show how strongly climate responds to changes in atmospheric composition, solar forcing, geography, and feedbacks within the oceans, atmosphere, biosphere, and cryosphere. By understanding ancient environments, we are better equipped to evaluate the causes, consequences, and likely trajectory of climate change today.