A pair of interacting hairpin oligonucleotide probes capable of hybridizing adjacently on a nucleic acid target sequence in fixed and permeabilized cells and interacting to generate a single-stranded initiator sequence capable of initiating a hybridization chain reaction or rolling circle amplification amplification, said probe pair comprising a first, arm-donating hairpin probe that is a stem-and-loop oligonucleotide having a double-stranded stem comprising two complementary arm sequences flanking a target-sequence-complementary single-stranded loop sequence, wherein one of the arm sequences (donating arm) includes a single-stranded extension, and a second, arm-acceptor hairpin probe that is a stem-and-loop oligonucleotide having a double-stranded stem comprising two complementary arm sequences flanking a single-stranded loop sequence, the loop and one arm comprising an HCR or RCA initiator sequence, the other arm having a single-stranded extension comprising at least a terminal target-sequence-complementary sequence, said second probe being capable of hybridizing to said target sequence adjacently to said first probe, wherein, when free in solution or bound non-specifically, the probes are capable of maintaining their stem-loop structures, wherein, hybridization of the first probe's loop sequence to the target sequence opens that probe's stem, but hybridization of second probe's terminal target-sequence-complementary sequence to the target sequence does not open that probe's stem, and wherein, if the first and second probes are correctly hybridized adjacently on the target sequence, the donating arm of the first probe is single-stranded and capable of hybridizing with the second probe's arm having the single-stranded extension, thereby rendering its HCR or RCA initiator sequence single-stranded. a first, arm-donating hairpin probe that is a stem-and-loop oligonucleotide having a double-stranded stem comprising two complementary arm sequences flanking a target-sequence-complementary single-stranded loop sequence, wherein one of the arm sequences (donating arm) includes a single-stranded extension, and a second, arm-acceptor hairpin probe that is a stem-and-loop oligonucleotide having a double-stranded stem comprising two complementary arm sequences flanking a single-stranded loop sequence, the loop and one arm comprising an HCR or RCA initiator sequence, the other arm having a single-stranded extension comprising at least a terminal target-sequence-complementary sequence, said second probe being capable of hybridizing to said target sequence adjacently to said first probe, wherein, when free in solution or bound non-specifically, the probes are capable of maintaining their stem-loop structures, wherein, hybridization of the first probe's loop sequence to the target sequence opens that probe's stem, but hybridization of second probe's terminal target-sequence-complementary sequence to the target sequence does not open that probe's stem, and wherein, if the first and second probes are correctly hybridized adjacently on the target sequence, the donating arm of the first probe is single-stranded and capable of hybridizing with the second probe's arm having the single-stranded extension, thereby rendering its HCR or RCA initiator sequence single-stranded.