Bubbles nucleating in supersaturated liquids grow by H2O diffusion in melts into bubbles (the number of H2O molecules in the bubbles increases) and expand by decompression (the number of H2O molecules in the bubbles does not change but the molecular volume increases). In this manner, “growth” (“grow”) and “expansion” (“expand”) are used differently for different phenomena in this chapter. Although growth and expansion simultaneously progress, their behavior will be separately understood by ignoring the other at first. Then, the two phenomena will be combined to understand the experimental results. In this chapter, important factors including time constants of various elemental processes and their dimensionless number ratios, such as the Pélclet number, which is the ratio of the diffusion to the viscosity-limited expansion rate, are discussed.