A method of detecting a target polynucleotide sequence in a given nucleic acid analyte characterised by the steps of: a. annealing the analyte to a single-stranded probe oligonucleotide A 0, which comprises a 3' end that is perfectly complementary to the target polynucleotide sequence, to create a first intermediate product in which the 3' end of A 0 forms a double-stranded complex with the analyte target sequence; b. pyrophosphorolysing the first intermediate product with a pyrophosphorolysing enzyme in the 3'-5' direction from the 3' end of A 0 to create partially digested strand A 1 and the analyte, wherein when A 0 erroneously hybridises with a non-target sequence, the pyrophosphorolysis reaction stops at any mismatches, preventing subsequent steps of the method from proceeding; wherein the pyrophosphorolysis reaction of (b) continues until A 1 lacks sufficient complementarity with the analyte or a target region therein to form a stable duplex and separates therefrom; c. (i) annealing A 1 to a single-stranded trigger oligonucleotide B and extending the A 1 strand in the 5'-3' direction against B, wherein B is comprised of an oligonucleotide region complementary to the 3' end of A 1 with a flanking oligonucleotide region at its 5' end which is not substantially complementary to A 0; or (ii) circularising A 1 through ligation of its 3' and 5' ends; or (iii) ligating the 3' end of A 1 to the 5' end of a ligation probe oligonucleotide C; in each case to create an oligonucleotide A 2; wherein the ligation is performed through addition of a further splint oligonucleotide D to which A 1 anneals prior to ligation, wherein D comprises an oligonucleotide region complementary to the 3' end of A 1 and a region complementary to either the 5' end of oligonucleotide C or to the 5' end of A 1; d. priming A 2 with at least one single-stranded primer oligonucleotide and creating multiple copies of A 2, or a region of A 2; and e. detecting a signal derived from the multiple copies and inferring therefrom the presence or absence of the polynucleotide target sequence in the analyte. a. annealing the analyte to a single-stranded probe oligonucleotide A 0, which comprises a 3' end that is perfectly complementary to the target polynucleotide sequence, to create a first intermediate product in which the 3' end of A 0 forms a double-stranded complex with the analyte target sequence; b. pyrophosphorolysing the first intermediate product with a pyrophosphorolysing enzyme in the 3'-5' direction from the 3' end of A 0 to create partially digested strand A 1 and the analyte, wherein when A 0 erroneously hybridises with a non-target sequence, the pyrophosphorolysis reaction stops at any mismatches, preventing subsequent steps of the method from proceeding; wherein the pyrophosphorolysis reaction of (b) continues until A 1 lacks sufficient complementarity with the analyte or a target region therein to form a stable duplex and separates therefrom; c. (i) annealing A 1 to a single-stranded trigger oligonucleotide B and extending the A 1 strand in the 5'-3' direction against B, wherein B is comprised of an oligonucleotide region complementary to the 3' end of A 1 with a flanking oligonucleotide region at its 5' end which is not substantially complementary to A 0; or (ii) circularising A 1 through ligation of its 3' and 5' ends; or (iii) ligating the 3' end of A 1 to the 5' end of a ligation probe oligonucleotide C; in each case to create an oligonucleotide A 2; wherein the ligation is performed through addition of a further splint oligonucleotide D to which A 1 anneals prior to ligation, wherein D comprises an oligonucleotide region complementary to the 3' end of A 1 and a region complementary to either the 5' end of oligonucleotide C or to the 5' end of A 1; d. priming A 2 with at least one single-stranded primer oligonucleotide and creating multiple copies of A 2, or a region of A 2; and e. detecting a signal derived from the multiple copies and inferring therefrom the presence or absence of the polynucleotide target sequence in the analyte.