This chapter details the physical phenomena that determine the extent to which sound propagates outdoors, reviews data that demonstrate their influence and describes methods for predicting their acoustic effects. Propagation of sound outdoors involves geometric spreading, air absorption, refraction associated with wind and temperature gradients and effects of atmospheric turbulence. When the source and receiver are relatively close to the ground, there are interactions with the ground, barriers and buildings, topography and vegetation. These interactions are compounded by various meteorological factors.