Nanophotonics enables precise control over light–matter interactions, though most established design frameworks for subwavelength nanoparticles rely on isotropic materials. Uniaxial and biaxial particles—common in natural and engineered systems—introduce new degrees of freedom that couple geometry and material properties, unlocking multispectral and directional response in previously unexplored spectral regions. We present a universal full-wave framework for eigenmodes and resonances in such nanoparticles. Closed-form solutions reveal axial-permittivity sum rules and anisotropy-induced symmetry breaking, producing resonance splitting and novel radiation patterns. Generalizing to ellipsoids enables geometric tuning of multispectral response, while analytic quality factors elucidate mode localization and loss. Full-wave simulations of h-BN and $\alpha$-MoO3 particles confirm the theory. This framework unifies the understanding of anisotropic nanostructures across optics, magnetism, and thermal transport, providing pathways to a new generation of photonic devices with tunable multispectral response and controlled emission with direct applications in sensing and imaging.

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• Light-matter interaction
• Magnetic anisotropy
• Photonics
• Scattering theory
• Thermal conductivity
• Crystal optics
• Nanoparticles
• Optical materials & elements