A probe for use in detecting a target analyte in a sample, wherein the probe provides or is capable of providing nucleic acid components sufficient to initiate a rolling circle amplification reaction, said probe being a nucleic acid construct comprising: (i) at least two domains each comprising a target binding domain capable of binding to the target analyte or to an intermediate molecule bound, directly or indirectly, to the target analyte; (ii) one or more domains together capable of providing a RCA template; and (iii) at least one domain capable of providing a primer for said RCA reaction that hybridises to a region of said RCA template and optionally a ligation template that templates the ligation of the RCA template, said domain comprising; (a) a region of complementarity to a sequence within the probe, such that it forms part of a hairpin structure that comprises a cleavage recognition site; or (b) a region of double stranded nucleic acid that comprises a cleavage recognition site, wherein at least part of the probe must be cleaved and/or unfolded to release said primer to enable said rolling circle amplification reaction, and wherein each domain capable of providing a RCA nucleic acid component is directly or indirectly attached to the target analyte via a target binding domain; the domain capable of providing the RCA template is adjacent to the domain capable of providing the ligation template; and cleavage of the probe releases said RCA nucleic acid components to enable a RCA reaction when the target binding domains are bound to the target analyte or intermediate molecule. (i) at least two domains each comprising a target binding domain capable of binding to the target analyte or to an intermediate molecule bound, directly or indirectly, to the target analyte; (ii) one or more domains together capable of providing a RCA template; and (iii) at least one domain capable of providing a primer for said RCA reaction that hybridises to a region of said RCA template and optionally a ligation template that templates the ligation of the RCA template, said domain comprising; (a) a region of complementarity to a sequence within the probe, such that it forms part of a hairpin structure that comprises a cleavage recognition site; or (b) a region of double stranded nucleic acid that comprises a cleavage recognition site, wherein at least part of the probe must be cleaved and/or unfolded to release said primer to enable said rolling circle amplification reaction, and wherein each domain capable of providing a RCA nucleic acid component is directly or indirectly attached to the target analyte via a target binding domain; the domain capable of providing the RCA template is adjacent to the domain capable of providing the ligation template; and cleavage of the probe releases said RCA nucleic acid components to enable a RCA reaction when the target binding domains are bound to the target analyte or intermediate molecule. (a) a region of complementarity to a sequence within the probe, such that it forms part of a hairpin structure that comprises a cleavage recognition site; or (b) a region of double stranded nucleic acid that comprises a cleavage recognition site, wherein at least part of the probe must be cleaved and/or unfolded to release said primer to enable said rolling circle amplification reaction, and wherein each domain capable of providing a RCA nucleic acid component is directly or indirectly attached to the target analyte via a target binding domain; the domain capable of providing the RCA template is adjacent to the domain capable of providing the ligation template; and cleavage of the probe releases said RCA nucleic acid components to enable a RCA reaction when the target binding domains are bound to the target analyte or intermediate molecule. each domain capable of providing a RCA nucleic acid component is directly or indirectly attached to the target analyte via a target binding domain; the domain capable of providing the RCA template is adjacent to the domain capable of providing the ligation template; and cleavage of the probe releases said RCA nucleic acid components to enable a RCA reaction when the target binding domains are bound to the target analyte or intermediate molecule.