Abstract: A method of characterizing one or more particles in a fluid, e.g. a liquid, using interferometric scattering optical (iSCAT) microscopy. The method involves illuminating a region of a fluid using an objective lens so that light is scattered by one or more particles in the fluid. The scattered light and reference light are captured using the objective lens and interfere at an imaging device. A succession of images of the interference is processed to determine image correlation values which define a gradual decorrelation over time from which a property of the particle(s) is determined.