The invention describes a method for the identification of compounds which bind to a target component of a biochemical system or modulate the activity of the target, by compartmentalizing the compounds into microcapsules together with the target, such that only a subset of the repertoire is represented in multiple copies in any one microcapsules; and identifying the compound which binds to or modulates the activity of the target. The invention enables the screening of large repertoires of molecules which can serve as leads for drug development.