An internally-constrained cyclic oligopeptide comprising a ring of at least six amino acids for specifically binding to a target ligand, wherein the ring comprises a plurality of amino acid domains, each domain comprising at least two epitope-forming amino acids, and two or more associating functional groups positioned so that they form one or more intra-cyclic associations; whereby the cyclic oligopeptide is constrained in a single conformation so that the epitope-forming amino acids form an epitope in each domain, each epitope being capable of specifically binding to a target ligand.