In noble metal nanoparticles light induces collective oscillations of electrons ("Plasmons"), leading to highly efficient light scattering (left). The scattering efficiency Qsca, and the resonance wavelength are dependent on the exact particle shape (center), particle size and on the surrounding medium, allowing the usage of plasmonic nanoparticles as non-bleaching bio-sensors. Because the scattered light is polarized (right), they can also be used as rotation sensors.