A smoke and fire barrier for installation in or over an opening and operable selectively to prevent the passage therethrough of smoke, comprises a stationary grill having apertures through which air can flow and sealing means operable to seal the apertures against the passage of smoke therethrough. The sealing means takes the form of a sliding plate which is positioned adjacent the grill and has apertures which register with the apertures of the grill when the smoke barrier is in an open configuration and lands between the apertures which register with the apertures of the grill when the smoke barrier is in closed configuration. The plate is slidable horizontally relative to the grill automatically in response to the presence of smoke and/or heat between a first position in which the smoke barrier is in its open configuration and a second position in which the smoke barrier is in its closed configuration.