The air around us is teeming with microscopic life. With every breath we take, we inhale thousands of bacteria, viruses and fungi. Scientists have known for almost 150 years that some of these airborne microbes cause disease in plants, domestic animals and people. More recently, we have learned that microorganisms may also affect the weather by allowing water to freeze at warmer temperatures and triggering the onset of precipitation. Astonishingly, a few of these microbes drift on large currents of air to cross oceans and continents. New tools and technology are helping investigators learn more about where these organisms originate, how they spread and the often unexpected ways in which they affect our world during their travels.