A method of selectively combining particulate material, comprising: (i) providing a layer of particulate material to a part bed; (ii) providing radiation to sinter a portion of the material of the layer; (iii) providing a further layer of particulate material overlying the prior layer of particulate material including the previously sintered portion of material; (iv) providing radiation to sinter a further portion of the material within the overlying further layer and to sinter said further portion with the previously sintered portion of material in the prior layer; (v) successively repeating blocks (iii) and (iv) to form a three-dimensional object; and wherein at least some of the layers of particulate material are pre-heated with a heater prior to sintering a portion of the material of the respective layer, the heater being configured to move relative to, and proximate, the particulate material.