1. A system for determining a measure of the concentration of analyte molecules or particles in a fluid sample, comprising: an assay substrate comprising a plurality of locations each comprising a binding surface forming or contained within such locations, wherein at least one binding surface comprises at least one analyte molecule or particle immobilized on the binding surface; at least one detector configured to address a plurality of the locations, able to produce at least one signal indicative of the presence or absence of an analyte molecule or particle at each location addressed and having an intensity varying with the number of analyte molecules or particles at each location; and at least one signal processor configured to determine from the at least one signal the percentage of said locations containing at least one analyte molecule or particle, and further configured to, based upon the percentage, either determine a measure of the concentration of analyte molecules or particles in the fluid sample based at least in part on the number of locations containing at least one analyte molecule or particle, or determine a measure of the concentration of analyte molecules or particles in the fluid sample based at least in part on an intensity level of the at least one signal indicative of the presence of a plurality of analyte molecules or particles.