In his thesis, the author explored optical methods to detect, quantify and characterize nanoparticles and viruses < 100 nm in size on a single particle basis, label-free and in real-time. The methods are based on optical interferometry, where the signal obtained from a single nanoparticle is proportional to the amplitude of the light scattered from the particle, rather than intensity. This results in weaker size dependence of the signal than for a technique that measures light scattering intensity, and therefore enables detection of smaller particles.