Abstract: The present disclosure, among other things, provides methodologies for quantifying targets of interest in samples by 1) capturing single target entities on individual solid phase particles in a manner that permits that those particles that contain captured targets to be optically distinguished from those that do not, and 2) optically analyzing the particles so that those with captured target entities are “counted”. In some embodiments, provided methods and compositions in the present application comprise a population of particles including one or more sub-populations distinguishable from one another.