While initial efforts for such low temperature densification concept were developed in the mid 70s, the topic has become increasingly prominent in the last decade. Currently, these low temperature methods can be classified into four main groups: (i) hydrothermal reaction sintering (HRS), (ii) hydrothermal hot pressing (HHP), (iii) pressure-assisted densification techniques: room-temperature densification (RTD), cold sintering (CS), warm press (WP), and finally no pressure assisted method called (iv) reactive hydrothermal liquid phase densification (rHLPD).