A) Overpressure of the hot, ionized interior (yellow) will cause the bubble to expand inside the natal cloud, sweeping up neutral gas in a dense shell (blue) (Spitzer 1978). If the expansion is supersonic, a shock front forms on the neutral side of the shell (dashed line). B) If the ionized gas contains a density gradient and/or the bubble is punctured, a flow of ionized gas will stream towards lower density and ultimately into the surrounding ISM, relieving the bubble from its pressure (Tenorio-Tagle 1979). C) Dust is dragged along in an ionized flow, where “upstream” dust approaching the ionizing source will be heated and halted by radiation pressure, resulting in a dust wave or bow wave (Ochsendorf et al. 2014; and see Sects. 3.2 and 4), which can be traced at mid-infrared wavelengths (red).