In the previous chapter, we learned by observing highly vesiculated volcanic rocks (pumice and scoria) that bubble formation in magma, i.e., the vesiculation of magma, has an extremely close relationship with appearances of eruptions. Moreover, we learned that the expansion of gas generated by magma vesiculation is a driving force for volcanic explosions and that magma vesiculation progresses by cooling and solidification of magma as well as plays a role in triggering eruptions. Thus, the conditions (to be precise, the necessary conditions) for magma vesiculation, which play an important role in volcanic eruptions, are dependent on the solubility of water (H2 O) (the major component of the gas phase that vesiculates in magma) in silicate melt. Then, how has the solubility of water in melt been understood? Moreover, what types of conditions (such as temperature and pressure, etc.) influence the solubility? In this chapter, we will increase our understanding of the solubility of water in magma.