Selective laser solidification apparatus, comprising; a powder bed (6;200;300) onto which a powder layer (400;500) can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a predefined gas flow direction, the gas flow unit comprising at least one gas outlet and at least one gas exhaust, the at least one gas outlet and the at least one gas exhaust being placed either side of the powder bed such that gas pumped from the at least one gas outlet passes to the at least one gas exhaust and provides a substantially planar flow of gas along the predefined gas flow direction, and a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form a required pattern (402;502), the laser scanning unit being arranged to form the required pattern from a plurality of stripes or stripe segments (S1-S12), each stripe or stripe segment being formed by advancing the laser beam along the stripe or stripe segment in a stripe formation direction, characterised in that the apparatus is arranged to deposit and selectively solidify a plurality of powder layers to form a three dimensional object, the laser scanning unit being arranged to form the required pattern of each powder layer from a plurality of stripes or stripe segments, wherein the stripe formation direction is anti-parallel to the gas flow direction or there is an oblique angle between the stripe formation direction and the gas flow direction such that the stripe formation direction is always at least partially opposed to the predefined gas flow direction for each stripe or stripe segment of each powder layer. a powder bed (6;200;300) onto which a powder layer (400;500) can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a predefined gas flow direction, the gas flow unit comprising at least one gas outlet and at least one gas exhaust, the at least one gas outlet and the at least one gas exhaust being placed either side of the powder bed such that gas pumped from the at least one gas outlet passes to the at least one gas exhaust and provides a substantially planar flow of gas along the predefined gas flow direction, and a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form a required pattern (402;502), the laser scanning unit being arranged to form the required pattern from a plurality of stripes or stripe segments (S1-S12), each stripe or stripe segment being formed by advancing the laser beam along the stripe or stripe segment in a stripe formation direction, characterised in that the apparatus is arranged to deposit and selectively solidify a plurality of powder layers to form a three dimensional object, the laser scanning unit being arranged to form the required pattern of each powder layer from a plurality of stripes or stripe segments, wherein the stripe formation direction is anti-parallel to the gas flow direction or there is an oblique angle between the stripe formation direction and the gas flow direction such that the stripe formation direction is always at least partially opposed to the predefined gas flow direction for each stripe or stripe segment of each powder layer.