There is provided a reconfigurable scanning electron microscope (RSEM) (100) comprising: (a) a gun assembly (110) and an associated electron optical column (120) for generating an electron beam (600), for demagnifying the electron beam (600) to generate an electron probe (C3) and for scanning the probe (C3) across a sample (190); (b) an electron detector (550) for detecting emissions from the sample (190) in response to scanned electron probe irradiation thereof and for generating a corresponding detected signal (Sd) indicative of the magnitude of the emissions; and (c) a display (170) for receiving the detected signal (Sd) and scanning signals (x, y) indicative of the position of the probe (C3) relative to the sample (190) for generating the image of the sample (190). The RSEM (100) is distinguished in that it further includes aperture bearing members (500, 520), each member (500, 520) including an associated electon-beam transmissive aperture, for at least partially gaseously isolating the gun assembly (110) and the electron optical column (110) from the sample (190), thereby enabling the RSEM (100) to be reconfigurable as a high-vacuum scanning electron microscope and also as an environmental scanning electron microscope, the RSEM (100) being reconfigurable to include no aperture members, one aperture member (500, 750) and a plurality of aperture members (500, 750; 520 850, 860).