When choosing a method for modeling optical properties, there are tradeoffs between accuracy and computational speed. No method is optimal for all scenarios, and the best choice depends on the properties of the nanoparticles, the spatial organization of the assembly, the optical properties to be computed, and the required accuracy. Some of the most flexible and widely used electromagnetic simulation approaches, finite element or finite difference time domain (FDTD) methods, discretize the differential form of Maxwell's equations. This discretization must be fine enough to resolve length scales much smaller than the nanoparticles considered, limiting the addressable system sizes while enabling treatment of arbitrary nanoparticle geometries and boundary conditions. As a result, these methods are most useful for small systems of nanoparticles with complex geometries, e.g. characterizing the chiral electric field produced around a small cluster of gold tetrahedral nanoparticles (Figure ). Finite element and FDTD methods have also been used to quantify resonant frequencies and near-field enhancements for other anisotropic shapes like cubic and spiked nanoparticles, nanoparticles near substrates, and small unit cells of periodic superlattices. To model assemblies involving more particles at reduced computational cost, Maxwell's equations can be solved in integral form by the discrete dipole approximation method, which discretizes only nanoparticle volumes, or boundary element methods, which discretize only nanoparticle surfaces. Efficient boundary element tools have been developed to study the effects of disorder on scattering, extinction, and near-field enhancement of plasmonic nanorod films with O(10 3 ) nanorods (Figure ), illustrating the scale of problems that such approaches can currently address.