A method for determining a measure of the concentration of analyte molecules or particles in a fluid sample, comprising: exposing a plurality of beads, that each include a binding surface having affinity for at least one type of analyte molecule or particle, to a solution containing or suspected of containing the at least one type of analyte molecules or particles; immobilizing analyte molecules or particles with respect to the plurality of beads such that a statistically significant fraction of the beads associate with a single analyte molecule or particle and a statistically significant fraction of the beads are free of any analyte molecule or particle; individually isolating at least a portion of the beads subjected to the immobilizing step from the remainder of said portion of the beads; individually addressing at least a portion of the beads and determining the number of said beads associated with an analyte molecule or particle by, in the case of direct detection, interrogating and/or detecting a molecule or moiety that is comprised in the analyte molecule or particle or, in the case of indirect detection, by interrogating and/or detecting a molecule or moiety that is comprised in a labelling agent that has been converted from a precursor labelling agent by exposure to the analyte molecule or particle; and determining a measure of the concentration of analyte molecules or particles in the fluid sample based at least in part on the number of beads subjected to the addressing step determined to be associated with an analyte molecule or particle; wherein the beads are each contained within a liquid droplet that is immiscible within a fluid in which the droplet is suspended during at least the addressing step; and wherein the step of individually addressing at least a portion of the beads comprises feeding the liquid droplets into a column such that they pass by an optical detection system in single file. exposing a plurality of beads, that each include a binding surface having affinity for at least one type of analyte molecule or particle, to a solution containing or suspected of containing the at least one type of analyte molecules or particles; immobilizing analyte molecules or particles with respect to the plurality of beads such that a statistically significant fraction of the beads associate with a single analyte molecule or particle and a statistically significant fraction of the beads are free of any analyte molecule or particle; individually isolating at least a portion of the beads subjected to the immobilizing step from the remainder of said portion of the beads; individually addressing at least a portion of the beads and determining the number of said beads associated with an analyte molecule or particle by, in the case of direct detection, interrogating and/or detecting a molecule or moiety that is comprised in the analyte molecule or particle or, in the case of indirect detection, by interrogating and/or detecting a molecule or moiety that is comprised in a labelling agent that has been converted from a precursor labelling agent by exposure to the analyte molecule or particle; and determining a measure of the concentration of analyte molecules or particles in the fluid sample based at least in part on the number of beads subjected to the addressing step determined to be associated with an analyte molecule or particle; wherein the beads are each contained within a liquid droplet that is immiscible within a fluid in which the droplet is suspended during at least the addressing step; and wherein the step of individually addressing at least a portion of the beads comprises feeding the liquid droplets into a column such that they pass by an optical detection system in single file.