Particle size is one of the most important parameters not only for description of fundamental properties of materials, but also within the biological systems as it can affect a number of key features and processes, such as drug targeting, delivery or distribution. Regarding nanosized particles, ISO standard ISO/TS 27687:2008 (ISO, 2008) provides their definition as an object with a size between 1 and 100 nm. In the EU, the respective Commission Recommendation (EC, 2011) defines a “nanomaterial” to be a “natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1–100 nm”. In an ideal case, the particles that are subject of characterization would be all homogeneous in shape and size with uniform properties. In this situation, any method measuring particle size would provide the same values of their diameters and the same particle size distribution, regardless of the principle of the measurement technique used. In the real world, however, most of the particles are non-spherical with different shapes that would undoubtedly influence their diameter determined using different methods (Merkus, 2009 and Barth, 1984). Techniques used for the particle size measurements are based on different principles. Here, visual or microscopic observation, the light scattering, ultrasound absorption, sedimentation velocity or Brownian motion can be named. One of the most relevant methods used is dynamic light scattering (DLS) as it provides measurements of particle sizes from the nanometer up to a few microns. This technique measures scattered light fluctuations caused by the Brownian motion which are then related to the size of the particles via translational diffusion coefficient D. Particle diameter thus obtained is referred to as a hydrodynamic diameter and stands for the diameter of a sphere that has the same translational diffusion coefficient as the particle. It is worth mentioning that the hydrodynamic diameter measured by DLS corresponds to the diameter of its dense core increased by the thickness of a layer of molecules adsorbed on its surface (for example surfactants) plus the thickness of the solvation, counter ion layer. The size of particles determined by DLS is z-averaged according to the scattering intensity of each particle fraction present in the sample. In practice relevant to biological applications, volume and even number distribution are more appropriate and they are smaller than z-averages.