and bound states in the continuum, [27,28] and have been used in applications like biosensing [29] and color printing. [30] Plasmonic assemblies have proven particularly powerful in the field of biosensing. The presence of a biomarker results in, e.g., the (dis-)assembly of solution-phase plasmonic particles, or to changes in the particle-spacing in an assembly. Both result in strong changes in the far-field optical response of the plasmonic assemblies that is most often monitored using spectroscopy in a UV-vis spectrometer. In solution-based sensors, clustering of metal nanoparticles can be induced by target molecules, shifting the optical extinction spectrum of the nanoparticle solution (Figure 1d). [31] Such clustering assays based on particle (dis-) assembly have resulted in colorimetric assays for a range of analytes including proteins, DNA/RNA sequences, heavy ions, and toxic compounds. [31,32] Single-molecule biosensors, on the other hand, detect analytes by probing the optical response of a single nanoparticle. An important advantage of single-molecule sensing is the ability to construct histograms of sensor properties at the single-molecule level, e.g., the induced plasmon shift and statistical parameters like the waiting-time between events. Single-molecule biosensing using plasmonic sensors has been demonstrated in 2012 by Figure 1. Reported applications of nanoparticle assemblies. a) Left: Calculated SERS enhancement factors for a sphere dimer with a 2 nm gap spacing. Right: A Raman spectra of methylene blue dispersed on a monolayer (black) and bilayer (red) of gold nanoparticles. Adapted with permission. [11] Copyright 2019, American Chemical Society. (https://pubs.acs.org/doi/10.1021/acsnano.9b04224) Further permissions related to this material should be directed to the ACS. b) Non-linear emission spectrum of noncentrosymmetric patterned gold nanorods. The third-harmonic generation (THG) peak is in green and the second-harmonic generation (SHG) is in orange. Top inset-scheme of the assembly's THG (green) and SHG (orange) stemming from the red excitation wavelength (ω). Bottom inset-electron microscopy image of the nanoassembly (scale bar 200 nm). Adapted with permission. [16] Copyright 2019, American Chemical Society. c) Examples of metamaterials comprised of plasmonic assemblies (scale bars are 500 nm). Reproduced with permission. [25] Copyright 2014, Springer Nature. d) Left: A cluster assay biosensor in which nanoparticle assembly is induced by a target molecule. Right: An example UV-vis spectrum of the gold nanoparticle monomers (red) and subsequent assemblies upon addition of target molecules (blue).