The past decade has been an exciting time to study atmospheres. Fundamental studies of Earth's general circulation and hydrological cycle have been stimulated by questions about past climates and the future impacts of humankind's activities. Long-term spacecraft and Earth-based observations of solar system planets have reinvigorated the study of comparative planetary climatology. The explosion in discoveries of planets outside our solar system has made atmospheric science integral to understanding the diversity of our solar system and the potential habitability of planets outside it. Thus, the AGU Chapman Conference: Crossing the Boundaries in Planetary Atmospheres—From Earth to Exoplanets, gathered Earth, solar system, and exoplanet scientists to share experiences, insights, and challenges from their individual disciplines and to discuss areas in which thinking broadly might enhance scientists' fundamental understanding of how atmospheres work.