Droplet libraries

A method for detecting target molecules in a fluid sample, the method comprising:

• providing, to a microfluidic device, a fluid sample comprising target molecules, a plurality of beads each comprising a bead-identifying label that identifies the bead and a reactive component for binding a target molecule thereto, and a detection component for binding to a target molecule that is bound to a bead;
• partitioning, in the microfluidic device, the fluid sample into plurality of separate and isolated partitions of fluid, wherein at least one of the plurality of partitions comprises an immunocomplex comprising a target molecule bound between a single bead, via an associated reactive component, and an associated detection component;
• and
• simultaneously detecting from within an intact partition of the plurality of partitions the bead-identifying label and an event associated with contents of the intact partition.