In the previous chapter, we learned that, for bubble generation, a supersaturated state is required. So, how do bubbles actually form in a supersaturated liquid? How do these bubbles grow to bubbles that we can see? To answer these questions and understand bubble formation, it is absolutely necessary for us to consider the meaning of the size and number of bubbles present in highly vesiculated volcanic rocks such as pumice and scoria, described in Chap. 1. These concepts are necessary for us to understand the dominant factors controlling eruption styles and intensity from the bubble texture in highly vesiculated volcanic rocks. In this chapter, we will understand the mechanisms of such bubble formation and behavior of bubbles in liquids based on both theories and experiments.