A method for the production of carbon nanotube structures comprising: (a) introducing a flow of metal catalyst or metal catalyst precursor into a temperature-controlled flow-through reactor; (b) exposing the flow of metal catalyst or metal catalyst precursor to a first temperature zone sufficient to generate particulate metal catalyst, wherein the first temperature zone includes a region of peak particle concentration, wherein the first temperature zone sufficient to generate particulate metal catalyst extends over at least the range 600 to 1100°C; (c) releasing an axial or radial flow of a source of carbon into the temperature-controlled flow-through reactor at a release point; (d) exposing the particulate metal catalyst and source of carbon to a second temperature zone downstream from the first temperature zone, wherein the second temperature zone is sufficient to produce carbon nanotube structures, wherein the second temperature zone sufficient to produce carbon nanotube structures extends over at least the range 900 to 1150°C, (e) exposing the particulate metal catalyst and source of carbon to a third temperature zone downstream from the second temperature zone, wherein the third temperature zone is sufficient to evaporate the particulate metal catalyst, wherein the third temperature zone sufficient to evaporate the particulate metal catalyst extends over at least the range 1150 to 1400°C; (f) exposing the particulate metal catalyst and source of carbon to a fourth temperature zone downstream from the third temperature zone, wherein the fourth temperature zone is sufficient to re-nucleate the particulate metal catalyst and to produce carbon nanotube structures, wherein the fourth temperature zone sufficient to re-nucleate the particulate metal catalyst and to produce carbon nanotube structures extends over at least the range 600 to 1150°C; and (g) discharging the carbon nanotube structures from a discharge outlet of the temperature-controlled flow-through reactor, wherein either the release point is between the beginning of the first temperature zone and the end of the second temperature zone or the method further comprises: (f') releasing an axial or radial flow of a source of sulphur into the temperature-controlled flow-through reactor at a release location, wherein the release location is at the fourth temperature zone. (a) introducing a flow of metal catalyst or metal catalyst precursor into a temperature-controlled flow-through reactor; (b) exposing the flow of metal catalyst or metal catalyst precursor to a first temperature zone sufficient to generate particulate metal catalyst, wherein the first temperature zone includes a region of peak particle concentration, wherein the first temperature zone sufficient to generate particulate metal catalyst extends over at least the range 600 to 1100°C; (c) releasing an axial or radial flow of a source of carbon into the temperature-controlled flow-through reactor at a release point; (d) exposing the particulate metal catalyst and source of carbon to a second temperature zone downstream from the first temperature zone, wherein the second temperature zone is sufficient to produce carbon nanotube structures, wherein the second temperature zone sufficient to produce carbon nanotube structures extends over at least the range 900 to 1150°C, (e) exposing the particulate metal catalyst and source of carbon to a third temperature zone downstream from the second temperature zone, wherein the third temperature zone is sufficient to evaporate the particulate metal catalyst, wherein the third temperature zone sufficient to evaporate the particulate metal catalyst extends over at least the range 1150 to 1400°C; (f) exposing the particulate metal catalyst and source of carbon to a fourth temperature zone downstream from the third temperature zone, wherein the fourth temperature zone is sufficient to re-nucleate the particulate metal catalyst and to produce carbon nanotube structures, wherein the fourth temperature zone sufficient to re-nucleate the particulate metal catalyst and to produce carbon nanotube structures extends over at least the range 600 to 1150°C; and (g) discharging the carbon nanotube structures from a discharge outlet of the temperature-controlled flow-through reactor,