In this book, the authors use molecular dynamics simulations to conduct a comprehensive study of the compression/superheating limit and phase transition of 2D (monolayer, bilayer, and trilayer) water/ice constrained in graphene nanocapillaries. When subjected to nanoscale confinement and under ultrahigh pressure, water and ice behave quite differently than their bulk counterparts, partly because the van der Waals pressure can spark a water-to-ice transformation, known as the metastability limit of two-dimensional (2D) liquids. From a mechanical standpoint, this liquid-to-solid transformation characterizes the compression limit (or metastability limit) of 2D water. The findings presented here could help us to better understand the phase behavior of 2D confined water/ice.