To　pursue　the　sensitive　and　efficient　detection　of　informative　biomolecules　for　bioanalysis　and　disease　diagnosis,　a　series　of　signal　amplification　techniques　have　been　put　forward.　Among　them,　hybridization　chain　reaction　(HCR)　is　an　isothermal　and　enzyme-free　process　where　the　cascade　reaction　of　hybridization　events　is　initiated　by　a　target　analyte,　yielding　a　long　nicked　dsDNA　molecule　analogous　to　alternating　copolymers.　Compared　with　conventional　polymerase　chain　reaction　(PCR)　that　can　proceed　only　with　the　aid　of　polymerases　and　complicated　thermal　cycling,　HCR　has　attracted　increasing　attention　because　it　can　occur　under　mild　conditions　without　using　enzymes.　As　a　powerful　signal　amplification　tool,　HCR　has　been　employed　to　construct　various　simple,　sensitive　and　economic　biosensors　for　detecting　nucleic　acids,　small　molecules,　cells,　and　proteins.　Moreover,　HCR　has　also　been　applied　to　assemble　complex　nanostructures,　some　of　which　even　act　as　the　carriers　to　execute　the　targeted　delivery　of　anticancer　drugs.　Recently,　HCR　has　engendered　tremendous　progress　in　RNA　imaging　applications,　which　can　not　only　achieve　endogenous　RNA　imaging　in　living　cells　or　even　living　animals　but　also　implement　imaging-guided　photodynamic　therapy,　paving　a　promising　path　to　promote　the　development　of　theranostics.　In　this　review,　we　begin　with　the　fundamentals　of　HCR　and　then　focus　on　summarizing　the　recent　advances　in　HCR-based　biosensors　for　biosensing　and　RNA　imaging　strategies.　Further,　the　challenges　and　future　perspective　of　HCR-based　signal　amplification　in　biosensing　and　theranostic　application　are　discussed.