A spark containment cap is disclosed for enclosing an end of a fastener protruding from a structure. The cap includes a cap body having an annular base terminating at a rim which surrounds an opening into an air cavity arranged to enclose the fastener end and having an aperture in fluid communication with the cavity. An annular bead of uncured sealing material is provided around the opening into the cavity, the bead of sealing material being curable to provide a seal between the cap body and the structure to seal a volume of gas within the cavity. A closure member has a closed configuration in which it seals the aperture, and an open configuration in which it does not seal the aperture.