The invention describes a method for the identification of compounds which bind to a target component of a bio-chemical system or modulate the activity of the target, comprising the steps of: a) compartmentalising the compounds into micro-capsules together with the target, such that only a subset of the repertoire is represented in multiple copies in any one microcapsule; and b) identifying the compound which binds to or modulates the activity of the target; wherein at least one step is performed under microfluidic control. The invention enables the screening of large repertoires of molecules which can serve as leads for drug development.