Supercooling is an undesired property of phase change materials due to the poorly predictable occurrence of crystallization during cooling. For such situations, the stored latent heat cannot be recovered which can be an issue for temperature-controlled applications. This review illustrates the techniques used for triggering crystallization in phase change materials having a supercooling property. The development of triggering devices should constitute a breakthrough for heat on demand applications, as heat can be released even when the temperature drops far below the liquidus temperature. Several techniques appear to be promising for nucleation triggering. They have been classified into two categories: passive (reduction of supercooling) or active (triggering of crystallization on demand) devices. They were accurately investigated for water freezing for: meteorological comprehension, food preservation or the pharmaceutical industry. In this paper, several nucleating agents (passive) have been explored, and most of them, added by 1 wt%, can decrease the supercooling degree by more than 90%. In addition, the heat would be immediately released on demand from a supercooled material by the use of seeding or electrofreezing (active methods). Solidification can also be externally triggered by the application of high pressure or ultrasonic waves (active). In addition to the analysis of the efficiency of the different techniques in terms of supercooling reduction, this review also discusses the solidification process at a microscopic scale.