This chapter discusses the sounds emitted by gas bubbles when they are generated underwater. Here, we define bubbles to be volumes of gas surrounded by liquid (in this case, taken to be water), having surface-tension forces (the so-called Laplace pressure ) generated by a single wall; they are distinguished from the soap bubbles familiar in children's games, where the volume of gas is surrounded by two gas/liquid boundaries. Compared to other acoustic sources, such as marine mammals, ships, and tectonic events, a single bubble may seem insignificant. Indeed, without ideal conditions, it can be difficult to observe the sound of a single bubble from a distance of more than a few tens of centimeters. However, natural processes rarely produce single bubbles and can generate them by the million, at which point the sound generated is significant. The formation of bubbles due to gas seeps, rainfall, and breaking waves is a major component of ambient noise in the marine environment and can even alter the propagation of sound waves from other sources. This chapter focuses on the passive emissions of bubbles as they are formed, released, or injected into water, with linear volume pulsations.