Many practical micro- and nanoscale systems in heterogeneous catalysis, electrolyzers or electroanalytical sensing can be represented as arrays of active sites distributed randomly on inert surfaces since this allows to decrease the amount of expensive materials and often provides even better performances than solid materials owing to enhanced micro- and nanoscale diffusion rates and/or more reactive surface atoms. Those may include partially blocked electrodes, membranes or non-(electro)chemically active substrates with active/catalytic nanosites dispersed on their surface to name a few. We develop approaches to model and characterize these complex systems as well as to understand better their intrinsic properties.