A method of primer extension, comprising: a) providing a support with multiple copies of immobilized forward primers, wherein all forward primers are identical in sequence or have an identical 3' portion, b) attaching a nucleic acid reverse strand to the support by hybridizing a first forward primer to a complementary forward primer-binding sequence on the nucleic acid reverse strand; c) generating an extended first forward strand that is a full-length complement of the reverse strand and is hybridized thereto, by extending the first forward primer using the reverse strand as template; d) separating at least part of the forward primer-binding sequence on the nucleic acid reverse strand and the forward primer portion of the first forward strand, e) hybridizing a second forward primer to the forward primer binding sequence on the nucleic acid reverse strand where the reverse strand is also hybridized to the first forward strand, f) amplifying the forward strand by one or more amplification cycles comprising steps (c) to (e) under isothermal conditions, wherein the second forward primer of step (e) of a first amplification cycle acts as the first forward primer of step (c) of a subsequent amplification cycle; and wherein a substantial proportion of reverse strands are hybridized to forward strands at all times during or between said one or more repetitions, and wherein said partial separation in step (d) is achieved by (i) using a recombinase, or (ii) adjusting the isothermal conditions to a temperature that is higher than the Tm of all forward primers, but lower than the Tm of the reverse strands, wherein the Tm of a reverse strand is the temperature at which half of the reverse strands in a clonal population of identical reverse strands are fully denatured from a perfectly complementary strand that is fully hybridized to the reverse strand across its entire length. a) providing a support with multiple copies of immobilized forward primers, wherein all forward primers are identical in sequence or have an identical 3' portion, b) attaching a nucleic acid reverse strand to the support by hybridizing a first forward primer to a complementary forward primer-binding sequence on the nucleic acid reverse strand; c) generating an extended first forward strand that is a full-length complement of the reverse strand and is hybridized thereto, by extending the first forward primer using the reverse strand as template; d) separating at least part of the forward primer-binding sequence on the nucleic acid reverse strand and the forward primer portion of the first forward strand, e) hybridizing a second forward primer to the forward primer binding sequence on the nucleic acid reverse strand where the reverse strand is also hybridized to the first forward strand, f) amplifying the forward strand by one or more amplification cycles comprising steps (c) to (e) under isothermal conditions, wherein the second forward primer of step (e) of a first amplification cycle acts as the first forward primer of step (c) of a subsequent amplification cycle; and wherein a substantial proportion of reverse strands are hybridized to forward strands at all times during or between said one or more repetitions, and (i) using a recombinase, or (ii) adjusting the isothermal conditions to a temperature that is higher than the Tm of all forward primers, but lower than the Tm of the reverse strands, wherein the Tm of a reverse strand is the temperature at which half of the reverse strands in a clonal population of identical reverse strands are fully denatured from a perfectly complementary strand that is fully hybridized to the reverse strand across its entire length.