A method for detecting the presence of a target nucleic acid of defined sequence in a sample which method comprises the steps: a) contacting said sample with: i. a first oligonucleotide probe; and ii. a nicking agent; wherein the first oligonucleotide probe comprises a first complementarity region capable of sequence specific hybridisation to the target nucleic acid and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the first oligonucleotide probe hybridises to the target nucleic acid in said sample and cleaves said first oligonucleotide probe to produce a first probe fragment; b) contacting said first probe fragment with: iii. a second oligonucleotide probe; and iv. a nicking agent; wherein the second oligonucleotide probe comprises a second complementarity region capable of sequence specific hybridisation to the first probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the second oligonucleotide probe hybridises to the first probe fragment and cleaves said second oligonucleotide probe to produce a second probe fragment; c) optionally contacting said second probe fragment with: v. a third oligonucleotide probe; and vi. a nicking agent; wherein the third oligonucleotide probe comprises a third complementarity region capable of sequence specific hybridisation to the second probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the third oligonucleotide probe hybridises to the second probe fragment and cleaves said third oligonucleotide probe to produce a third probe fragment; d) optionally repeating step c) n times, wherein n is a positive integer, using fourth and subsequent sequential (3+n)th oligonucleotide probe(s) to produce fourth and subsequent sequential (3+n)th probe fragment(s); and e) detecting the presence of the probe fragment produced at the end of step (b) and/or step (c) and/or step (d), wherein one or more of said detected probe fragment(s) is not capable of sequence specific hybridisation to the complementarity region of any of the preceding oligonucleotide probes to form a site that is specifically recognised and cleaved by any of the said nicking agent(s) and/or none of the probe fragments produced is capable of sequence specific hybridisation to the first complementarity region of the first oligonucleotide probe, and wherein the presence of said detected probe fragment(s) indicates the presence of the target nucleic acid in said sample. a) contacting said sample with: i. a first oligonucleotide probe; and ii. a nicking agent; wherein the first oligonucleotide probe comprises a first complementarity region capable of sequence specific hybridisation to the target nucleic acid and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the first oligonucleotide probe hybridises to the target nucleic acid in said sample and cleaves said first oligonucleotide probe to produce a first probe fragment; i. a first oligonucleotide probe; and ii. a nicking agent; b) contacting said first probe fragment with: iii. a second oligonucleotide probe; and iv. a nicking agent; wherein the second oligonucleotide probe comprises a second complementarity region capable of sequence specific hybridisation to the first probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the second oligonucleotide probe hybridises to the first probe fragment and cleaves said second oligonucleotide probe to produce a second probe fragment; iii. a second oligonucleotide probe; and iv. a nicking agent; c) optionally contacting said second probe fragment with: v. a third oligonucleotide probe; and vi. a nicking agent; wherein the third oligonucleotide probe comprises a third complementarity region capable of sequence specific hybridisation to the second probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the third oligonucleotide probe hybridises to the second probe fragment and cleaves said third oligonucleotide probe to produce a third probe fragment; v. a third oligonucleotide probe; and vi. a nicking agent; d) optionally repeating step c) n times, wherein n is a positive integer, using fourth and subsequent sequential (3+n)th oligonucleotide probe(s) to produce fourth and subsequent sequential (3+n)th probe fragment(s); and e) detecting the presence of the probe fragment produced at the end of step (b) and/or step (c) and/or step (d), wherein one or more of said detected probe fragment(s) is not capable of sequence specific hybridisation to the complementarity region of any of the preceding oligonucleotide probes to form a site that is specifically recognised and cleaved by any of the said nicking agent(s) and/or none of the probe fragments produced is capable of sequence specific hybridisation to the first complementarity region of the first oligonucleotide probe, and wherein the presence of said detected probe fragment(s) indicates the presence of the target nucleic acid in said sample.