A method for spatially tagging nucleic acids of a biological specimen comprising: (a) providing a solid substrate comprising captures probes comprising a first species of capture probes and a second species of capture probes immobilized such that both the first species and the second species of capture probes occupy a distinct position on the solid substrate, wherein the capture probes of the first species and the second species are a primer for an extension reaction and wherein: the capture probes of the first and second species comprise a nucleic acid molecule comprising: (i) a cleavage domain for releasing the capture probes from the surface of the solid substrate, and (ii) a positional domain that corresponds to the position of the capture probes on the solid substrate, and wherein the first species of capture probes comprises a capture domain comprising a poly DNA oligonucleotide; and the second species of capture probes comprises a capture domain for a target nucleic acid; (b) contacting the solid substrate with the biological specimen; and (c) simultaneously releasing the capture probes from the surface of the solid substrate under conditions that allow a nucleic acid of the biological specimen to hybridise to the capture domains of the capture probes and extending the capture probes using the nucleic acid hybridised to the capture probes as extension templates to produce extended capture probes, wherein step (c) comprises contacting said solid substrate with an aqueous reaction mixture comprising: (i) a polymerase enzyme capable of extending the capture probes using the nucleic acid hybridised to the capture probes as an extension template to produce the extended capture probes; and (ii) a means for releasing the capture probes from the surface of the solid substrate. (a) providing a solid substrate comprising captures probes comprising a first species of capture probes and a second species of capture probes immobilized such that both the first species and the second species of capture probes occupy a distinct position on the solid substrate, wherein the capture probes of the first species and the second species are a primer for an extension reaction and wherein: the capture probes of the first and second species comprise a nucleic acid molecule comprising: (i) a cleavage domain for releasing the capture probes from the surface of the solid substrate, and (ii) a positional domain that corresponds to the position of the capture probes on the solid substrate, and wherein the first species of capture probes comprises a capture domain comprising a poly DNA oligonucleotide; and the second species of capture probes comprises a capture domain for a target nucleic acid; (i) a cleavage domain for releasing the capture probes from the surface of the solid substrate, and (ii) a positional domain that corresponds to the position of the capture probes on the solid substrate, and wherein the first species of capture probes comprises a capture domain comprising a poly DNA oligonucleotide; and the second species of capture probes comprises a capture domain for a target nucleic acid; wherein the first species of capture probes comprises a capture domain comprising a poly DNA oligonucleotide; and the second species of capture probes comprises a capture domain for a target nucleic acid; (b) contacting the solid substrate with the biological specimen; and (c) simultaneously releasing the capture probes from the surface of the solid substrate under conditions that allow a nucleic acid of the biological specimen to hybridise to the capture domains of the capture probes and extending the capture probes using the nucleic acid hybridised to the capture probes as extension templates to produce extended capture probes, wherein step (c) comprises contacting said solid substrate with an aqueous reaction mixture comprising: (i) a polymerase enzyme capable of extending the capture probes using the nucleic acid hybridised to the capture probes as an extension template to produce the extended capture probes; and (ii) a means for releasing the capture probes from the surface of the solid substrate. (i) a polymerase enzyme capable of extending the capture probes using the nucleic acid hybridised to the capture probes as an extension template to produce the extended capture probes; and (ii) a means for releasing the capture probes from the surface of the solid substrate.