When the electromagnetic energy (i.e., light) absorbed by nanoparticles is dissipated as heat, the temperature of the nanoparticle and the surrounding medium rises. This phenomenon is known as the photothermal effect. Plasmonic nanoparticles are excellent light absorber with high absorption cross sections. The strong absorption is due to the resonant free electron oscillations (also known as the plasmon resonance) in these nanoparticles driven by the incident light or electromagnetic field. The photothermal effect of plasmonic nanoparticles is promising in many applications such as solar energy harvesting , solar desalination , thermal catalysis , photoacoustic imaging , sensing , optofluidic tweezers , etc. It has been shown that plasmonic coupling in assemblies of nanoparticles results in an enhancement of the individual nanoparticle absorption intensity, which in turn enhances the photothermal properties. For light harvesting applications, high absorption intensity (or absorptance in the case of a film) should also be accompanied by low scattering (or reflectance in the case of a film) for the minimization of the energy loss. 3D and 2D assemblies of plasmonic nanoparticles with small (up to a few nm) interparticle gaps are thus excellent light harvesters. Zhou et al. demonstrated hybrid porous structures with assemblies of Al nanoparticles inside for complete absorption of light without scattering (or reflection), Figure . These structures achieved significant enhancement in the water evaporation rate for the application in solar desalination. Similarly, 3D plasmonic nanoparticle assemblies exhibit broadband absorption for greater coverage of the solar spectrum. Also, strong absorption in the near-infrared and infrared is advantageous for photoacoustic imaging in biological systems. The broadening and the red-shift of the plasmon band in the nanoparticle assemblies is a result of the plasmonic coupling. As shown by Zhu et al., nanogap Au nanoparticle assemblies with high infrared absorptance provide a strong photoacoustic signal for imaging of tumors, Figure . The potential of plasmonic nanoparticle assemblies (or clusters) in photoacoustic imaging has also been demonstrated in various other works. Photothermal release of drugs in target cells is another important medical application which can be further enhanced by the clustering effect. Goodman et al. showed that DNA release from nanoparticles in gene therapy by laser illumination has two distinctly different mechanisms for continuous wave (CW) laser and pulsed laser. While CW laser illumination drives the release of dehybridized single-stranded DNA though a photothermal process, pulsed-laser illumination results in DNA release by the breakage of the Au-S bond. When Au nanoparticles (or any other plasmonic nanoparticles) form hollow clusters obtained by Pickering emulsion mediated self-assembly, the plasmonic coupling and the hollow structure together lead to extreme broadening of the optical extinction spectra, Figure ). Such clusters can be excellent broadband solar energy harvesters as the spectrum covers the entire solar energy range. Song et al. demonstrated an interesting anticounterfeiting application of self-assembled Au nanorods based on the photothermal effect . Depending on the self-assembly structure as shown in Figure , the photothermal effect can activate thermochromic dyes to show encoding patterns in 20 s that again becomes invisible 10 minutes after laser illumination is stopped. Reprinted with permission © 2007 Nature Publishing Group (b) Au nanoparticle clusters have red-shifted and stronger plasmon bands (absorption) for enhanced photoacoustic imaging. Reprinted with permission © 2020, American Chemical Society (c) Au nanoparticle hollow clusters obtained by Pickering emulsion mediated self-assembly procedure have nearly complete absorption of the visible spectrum to be used for light harvesting applications. Reprinted with permission © 2015 John Wiley & Sons. (d) Self-assembled Au nanorods of different configurations can be used for photothermal anticounterfeiting patterns that are activated by laser illumination due to the presence of thermochromic dyes. Reprinted with permission © 2020 John Wiley & Sons.