The present invention relates to systems and methods for detecting analyte molecules or particles in a fluid sample and in some cases, determining a measure of the concentration of the molecules or particles in the fluid sample. Methods of the present invention may comprise immobilizing a plurality of analyte molecules or particles with respect to a plurality of capture objects. At least a portion of the plurality of capture objects may be spatially separated into a plurality of locations. A measure of the concentration of analyte molecules in a fluid sample may be determined, at least in part, on the number of reaction vessels comprising an analyte molecule immobilized with respect to a capture object. In some cases, the assay may additionally comprise steps including binding ligands, precursor labeling agents, and/or enzymatic components.