Cross-linking is a process where a polymer chain on the shell material is linked to another polymer chain through a chemical bond. Cross-linking provides better mechanical strength to the shell materials. Al-Shannaq et al. reported adding a small amount of cross-linker to the system improves the surface morphology of the microcapsules to be smooth and compact and produces microcapsules with a much higher PCM content. Hawlader et al. [11] reported an example of cross-linking in synthesis using complex-coacervation technique of Gum-Arabica/Gelatin capsules. Authors describe that, carboxylate functional group on gelatin molecule with a formaldehyde molecule, activates carboxylic group to nucleophillic attack. A reaction of such groups on other gelatin molecules with the first activated carboxylate cross-links the gelatin. This hardening reaction is catalyzed by OHsingle bond (alkaline condition). The change in pH value results in ionization and deionization of the functional groups . For example carboxylic acid group (COOH) are not ionized at low pH of less than 5.5, whereas charged COOsingle bond groups repulse each other at high pH of 5.5–7.4  Increasing the amount of cross-linking agent leads to an increase in the encapsulation efficiency. However beyond an optimum 8 ml of cross-linking agent, further increase did not result in much improvement in encapsulation efficiency ]. Alay et al.  reported that cross-linker type used for shell polymerization had an impact on thermal properties, particle size and structure of microcapsules. In the same study they also used polyurethane as a cross-linker to fix the microcapsules to cotton and polyester in thermo-regulated fabrics. Table 2 gives a list of commonly used cross-linking agents along with related shell materials and synthesis techniques.