Abstract: Methods and systems for capture object-based assays, including for determining a measure of the concentration of an analyte molecule or particle in a fluid sample, are described. The methods and systems may relate to high sensitivity detection of analytes, sometimes using assay conditions and sample handling that result in the capture and detection of a high percentage of the analyte molecules or particles in a fluid sample using relatively few capture objects. Apparatuses and methods for immobilizing capture objects with respect to assay sites, in some instances with unexpectedly high efficiencies are also described. Some such apparatuses involve the use of force fields and fluid meniscus forces, alone or in combination, to facilitate or improve capture object immobilization.