Abstract An intuitive treatment of Brownian motion is described, mainly based on the Langevin and generalized Smoluchowsky equation. It is assumed that spherical particles undergo Brownian forces as well as mutual hydrodynamic and intreparticle interactions when in thermal equilibrium. The velocity auto-correlation function summarizes the temporal behavior and the effects on the function due to Brownian forces and particle interactions are represented by parameters called a Brownian relaxation time, τ B, a hydrodynamic relaxation time, τ H, and an interaction relaxation time, τ I. The function is closely related to the mean square displacement of the particles. The mean square displacement is directly related to the average scattering of light from Brownian particles. The resulting effect is, in the frequency domain, the frequency broadening and, in the time domain, the intensity fluctuation. The latter is more suited in experimental observations of the temporal behavior of Brownian motion, although the detector size must be limited within the coherence area.