This invention concerns an additive manufacturing apparatus for building a part by selectively consolidating flowable material in a layer-by-layer process comprising a build chamber (101) for building the part, a module (105, 106) for providing a focussed energy beam for consolidating flowable material in the build chamber, a gas flow circuit for generating a gas flow through the build chamber (101). At least one filter assembly (200, 201) may be arranged in the gas flow circuit, the or each filter assembly (200, 201) having associated therewith a valve (V-4, V-8) operable to seal the gas circuit upstream from the filter assembly (200, 201) and a valve (V-5, V-9) operable to seal the gas flow circuit downstream of the filter assembly (200, 201), the arrangement allowing a filter element (E-5, E-7) of the filter assembly (200, 201) to be changed whilst maintaining a controlled atmosphere in the build chamber (101). The apparatus may further comprise a purging device (210) configured to purge air from the or each filter assembly (200, 201) when the valves (V-4, V-5; V-8, V-9) associated with that filter assembly (200, 201) have sealed the filter assembly (200, 201) from the gas flow. Alternatively or additionally, a controller (131) may be arranged to control the valves (V-4, V-5; V-8, V-9) associated with the or each filter assembly (200, 201) such that the filter assemblies (200, 201) are closed off to gas flow from the build chamber (101) during a period in which the inert gas atmosphere in the build chamber (101) is compromised as a result of opening the door to the build chamber (101). The apparatus may comprise a pair of filter assemblies (200, 201) arranged in parallel within the gas flow circuit, the arrangement allowing a filter element (E-5, E-7) of the each filter assembly (200, 201) to be changed during a build by allowing the filter element (E-5, E-7) of one filter assembly (200, 201) to be changed whilst the filter element (E-5, E-7) of the other filter assembly (200, 201) is connected to filter particulates from the gas flow. A monitoring device (I-3, I-5) may be provided for detecting a property associated with the gas flow and a controller (131) arranged to control the valves to switch the filter assembly (200, 201) connected in line with the gas flow based upon signals from the monitoring device (I-3, I-5).