The present invention relates to the use of nucleic acid probes to identify analytes in a sample. In the preferred embodiments, metastable nucleic acid monomers are provided that associate in the presence of an initiator nucleic acid. Upon exposure to the initiator, the monomers self-assemble in a hybridization chain reaction. The initiator nucleic acid may be, for example, a portion of an analyte to be detected or may be part of an initiation trigger such that it is made available in the presence of a target analyte.