Phase change materials (PCMs) can store thermal energy as latent heat through phase transitions. PCMs using the solid-liquid phase transition offer high 100–300 J g−1 enthalpy at constant temperature. However, pure compounds suffer from leakage, incongruent melting and crystallization, phase separation, and supercooling, which limit their heat storage capacity and reliability during multiple heating-cooling cycles. An appropriate approach to mitigating these drawbacks is the construction of composites as shape-stabilized phase change materials which retain their macroscopic solid shape even at temperatures above the melting point of the active heat storage compound. Shape-stabilized materials can be obtained by PCMs impregnation into porous matrices. Porous silica nanomaterials are promising matrices due to their high porosity and adsorption capacity, chemical and thermal stability and possibility of changing their structure through chemical synthesis. This review offers a first in-depth look at the various methods for obtaining composite PCMs using porous silica nanomaterials, their properties, and applications. The synthesis and properties of porous silica composites are presented based on the main classes of compounds which can act as heat storage materials (paraffins, fatty acids, polymers, small organic molecules, hydrated salts, molten salts and metals). The physico-chemical phenomena arising from the nanoconfinement of phase change materials into the silica pores are discussed from both theoretical and practical standpoints. The lessons learned so far in designing efficient composite PCMs using porous silica matrices are presented, as well as the future perspectives on improving the heat storage materials. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ). Phase change materials (PCMs) can store thermal energy as latent heat through phase transitions. PCMs using the solid-liquid phase transition offer high 100–300 J g −1 enthalpy at constant temperature. However, pure compounds suffer from leakage, incongruent melting and crystallization, phase separation, and supercooling, which limit their heat storage capacity and reliability during multiple heating-cooling cycles. Applications of thermal energy storage range from passively heated and cooled building, waste heat reutilization and renewable energy storage to creating better thermal insulation of transplant organs, food or electronics and battery heat management. One of the most promising technologies for thermal energy storage is comprised of phase change materials (PCMs), which can reversibly store large amounts of heat and cold at constant operating temperature. The use of pure compounds as PCMs has a number of drawbacks, such as leakage during use, poor reliability during multiple operating cycles, corrosiveness, flammability, etc. Liquid water has one of the highest specific heats, at 4.184 J g −1 K −1 . However, water remains liquid in a short temperature range, so cheaper materials with high thermal stability ranges but low c p , such as concrete or rock, are also employed for sensible heat storage [ 3 ]. Latent heat storage relies on the energy exchange which takes place during a phase transition such as melting/crystallization or evaporation/condensation. Latent heat storage can accommodate large heat storage capacities at constant temperature. Materials used for latent storage are called phase change materials (PCM). Finally, chemical heat storage involves the reaction enthalpy to be used for heat storage. Specific heat capacity, the operating temperature range, cost, and density are thus the most important characteristics of a sensible heat storage material ( Table 1 ). In contrast, latent heat storage offers higher energy densities at constant operating temperature. PCMs for latent storage can in theory use any phase transition between solid, liquid and gas phases. However, the change in volume accompanying the gas phase is very large, greatly reducing the volumetric storage capacity of such a system. In practice, only transitions between solid and liquid phases are employed. High volume change can lead in time to decreased contact area between the thermal energy storage materials and the heat transfer surfaces, thus diminishing both the energy and power densities. Other problems such as leakage are also associated with this volume change. The materials should also have low vapor pressure inside the operating temperature range. While encapsulation into porous silica can control the leakage of molten PCMs, evaporation remains a potential problem, especially in the case of organic materials. High crystallization rates and low supercooling degrees are also desired, as the crystallization process is hampered due to nanoconfinement effects [ 63 ]. Metals, Alloys and Elemental PCMs Metals and alloys can be used as PCMs. While most metals have low gravimetric heat of fusion, their high density makes them attractive for applications requiring high volumetric heat storage. There are only a few reports to date on the synthesis and characterization of metal-porous silica composites. A mixture of Ge and Sn embedded into a silica matrix was found to exhibit different melting and crystallization behavior, depending on heating and cooling rate [ 200 ]. Under fast temperature change rates the nanocrystals formed an alloy, while slow rates resulted in a more thermodynamically favored state of separated metal phases. More complex determinations are based on geometric considerations of the pore shape or volume, determined from porosimetry measurements. Both methods allow for the determination of theoretical enthalpy lost due to the non-melting layer and the enthalpy of the nanoconfined phase. In practice, a part of the pore volume remains empty due to the difference between the molar volume of the