An additive manufacturing system comprising: a build surface (3, 27, 43, 63, 83, 101, 143, 162, 203, 228, 266, 303, 322, 362, 402, 502); one or more laser energy sources (4, 21, 40, 61, 81, 108, 141, 161, 201, 223, 264, 306, 324, 364, 404); an optics assembly (1, 20) movable relative to the build surface and configured to direct laser energy from the one or more laser energy sources toward the build surface, wherein exposure of a layer of material on the build surface to the laser energy melts at least a portion of the layer of material and being characterised by: a gas flow head (2, 23, 60, 80, 100, 120, 140, 160, 200, 220, 260, 300, 320, 340, 360, 400, 504) coupled to the optics assembly and comprising a housing defining a partially enclosed volume (30, 48, 67, 84, 222, 333) between the optics assembly and the build surface, the gas flow head further comprising: a gas inflow (31, 70, 85, 365, 508) through which a supply gas (25, 46, 68, 86, 106. 145, 168, 225, 271, 308, 328, 347, 410) flows into the gas flow head, a gas outflow (32, 72, 88, 367, 510) through which a return gas (26, 47, 71, 87, 107, 146, 169, 226, 272, 309, 330, 348) flows out of the gas flow head, and an aperture (24, 41, 66, 91, 104, 144, 170, 206, 231, 261, 305, 327, 343, 371, 406, 407, 506) arranged to permit transmission of the laser energy through the gas flow head to the build surface, wherein the supply gas and return gas define a gas flow profile (45, 166, 167, 276, 310) within the gas flow head during operation of the gas flow head; wherein, in use, a first flow rate of the supply gas into the gas flow head is less than a second flow rate of the return gas out of the gas flow head during operation of the gas flow head, and a difference between the first flow rate and the second flow rate causes a scavenge gas (64, 65, 150, 205, 233, 273, 313) to flow into the gas flow head. a build surface (3, 27, 43, 63, 83, 101, 143, 162, 203, 228, 266, 303, 322, 362, 402, 502); one or more laser energy sources (4, 21, 40, 61, 81, 108, 141, 161, 201, 223, 264, 306, 324, 364, 404); an optics assembly (1, 20) movable relative to the build surface and configured to direct laser energy from the one or more laser energy sources toward the build surface, wherein exposure of a layer of material on the build surface to the laser energy melts at least a portion of the layer of material and a gas flow head (2, 23, 60, 80, 100, 120, 140, 160, 200, 220, 260, 300, 320, 340, 360, 400, 504) coupled to the optics assembly and comprising a housing defining a partially enclosed volume (30, 48, 67, 84, 222, 333) between the optics assembly and the build surface, the gas flow head further comprising: a gas inflow (31, 70, 85, 365, 508) through which a supply gas (25, 46, 68, 86, 106. 145, 168, 225, 271, 308, 328, 347, 410) flows into the gas flow head, a gas outflow (32, 72, 88, 367, 510) through which a return gas (26, 47, 71, 87, 107, 146, 169, 226, 272, 309, 330, 348) flows out of the gas flow head, and an aperture (24, 41, 66, 91, 104, 144, 170, 206, 231, 261, 305, 327, 343, 371, 406, 407, 506) arranged to permit transmission of the laser energy through the gas flow head to the build surface, wherein the supply gas and return gas define a gas flow profile (45, 166, 167, 276, 310) within the gas flow head during operation of the gas flow head; a gas inflow (31, 70, 85, 365, 508) through which a supply gas (25, 46, 68, 86, 106. 145, 168, 225, 271, 308, 328, 347, 410) flows into the gas flow head, a gas outflow (32, 72, 88, 367, 510) through which a return gas (26, 47, 71, 87, 107, 146, 169, 226, 272, 309, 330, 348) flows out of the gas flow head, and an aperture (24, 41, 66, 91, 104, 144, 170, 206, 231, 261, 305, 327, 343, 371, 406, 407, 506) arranged to permit transmission of the laser energy through the gas flow head to the build surface, wherein the supply gas and return gas define a gas flow profile (45, 166, 167, 276, 310) within the gas flow head during operation of the gas flow head; wherein, in use, a first flow rate of the supply gas into the gas flow head is less than a second flow rate of the return gas out of the gas flow head during operation of the gas flow head, and a difference between the first flow rate and the second flow rate causes a scavenge gas (64, 65, 150, 205, 233, 273, 313) to flow into the gas flow head.