Methods, reagents, compositions, and kits for reactivity-dependent polymerase chain reaction (RD-PCR) and interaction-dependent polymerase chain reaction (ID-PCR) are provided herein. RD-PCR is a technique useful for determining whether a reactive moiety can form a covalent bond to a target reactive moiety, for example, in screening a library of candidate reactive moieties for reactivity with a target reactive moiety, and in identifying an enzyme substrate, for example, in protease substrate profiling. ID-PCR is a technique useful for determining whether a ligand can non-covalently bind to a target molecule, for example, in screening a library of candidate ligands for non-covalent interaction with a target molecule. RD-PCR and ID-PCR are also useful in detecting the presence of an analyte or an environmental condition.