Metal nanoparticles’ (NP) covalent assemblies exhibit interesting structural, electronic, and photonic features of interest for applications in catalysis. In these structures, the ligands play a fundamental role on constructing the NP network and defining their chemical environment. Two types of strategies to produce NP assemblies are discussed in this chapter: (i) the direct cross-linking method, which is simple and well-controlled, and involves a chemical reaction between the metal nanoparticle surface and the ligand; and (ii) the indirect cross-linking method in which the chemical reaction necessary for network building does not directly involve the surface of the metallic nanoparticle. Additionally, the formation of reversible covalent networks is also discussed, which allows switching between a covalent network of NP and isolated NP by applying diverse stimuli. Generally, there is a lack of attention paid on the catalytic application of metal NP covalent assemblies, and this despite of the interesting properties of such assemblies for catalysis, for instance: (i) a confined environment, (ii) the possibility to finely tune the metal/ligand interaction, and (iii) the potential robustness of the structure. Various reactions catalyzed by NP networks have been investigated such as reduction, oxidation, or water-splitting, most of them focusing on Au NP, and also few other metals (Ag, Pd, Pt, Ru). As illustrated in some cases, the organized networks show better catalytic performances than dispersed or aggregated NP due to stability or confinement effect.