A method for spatially tagging nucleic acids of a biological specimen comprising: (a) providing a solid substrate on which multiple species of capture probes are immobilized such that each species occupies a distinct position on the solid substrate, wherein each capture probe of the multiple species is a primer for an extension reaction and wherein each capture probe of the multiple species is immobilized at its 3' end and comprises a nucleic acid molecule comprising in a 5' to 3' direction: (i) a positional domain that corresponds to a position of the capture probe on the solid substrate; (ii) a capture domain; and (iii) a cleavage domain for releasing the capture probe from the solid substrate; (b) contacting the solid substrate with the biological specimen; and (c) simultaneously releasing the multiple species of capture probes from the solid substrate under conditions that allow a nucleic acid of the biological specimen to hybridise to a capture domain of the capture probe and extending the capture probe using the nucleic acid hybridised to the capture probe as an extension template to produce an extended capture probe, wherein step (c) comprises contacting the solid substrate with an aqueous reaction mixture comprising: (i) a polymerase capable of extending the capture probe using the nucleic acid hybridised to the capture probe as an extension template to produce the extended capture probe; and (ii) a means for releasing the multiple species of capture probes from the solid substrate. (a) providing a solid substrate on which multiple species of capture probes are immobilized such that each species occupies a distinct position on the solid substrate, wherein each capture probe of the multiple species is a primer for an extension reaction and wherein each capture probe of the multiple species is immobilized at its 3' end and comprises a nucleic acid molecule comprising in a 5' to 3' direction: (i) a positional domain that corresponds to a position of the capture probe on the solid substrate; (ii) a capture domain; and (iii) a cleavage domain for releasing the capture probe from the solid substrate; (i) a positional domain that corresponds to a position of the capture probe on the solid substrate; (ii) a capture domain; and (iii) a cleavage domain for releasing the capture probe from the solid substrate; (b) contacting the solid substrate with the biological specimen; and (c) simultaneously releasing the multiple species of capture probes from the solid substrate under conditions that allow a nucleic acid of the biological specimen to hybridise to a capture domain of the capture probe and extending the capture probe using the nucleic acid hybridised to the capture probe as an extension template to produce an extended capture probe, wherein step (c) comprises contacting the solid substrate with an aqueous reaction mixture comprising: (i) a polymerase capable of extending the capture probe using the nucleic acid hybridised to the capture probe as an extension template to produce the extended capture probe; and (ii) a means for releasing the multiple species of capture probes from the solid substrate. (i) a polymerase capable of extending the capture probe using the nucleic acid hybridised to the capture probe as an extension template to produce the extended capture probe; and (ii) a means for releasing the multiple species of capture probes from the solid substrate.