The measurement techniques employed to obtain the shielding effectiveness (SE) of enclosures using the current version of IEEE 299.1 only apply to enclosures of side dimension greater than 0.1m, and are also lacking in a full analysis of the field distributions within the enclosure. The work presented here uses the IEEE 299.1 draft standard as a base and investigates different methods of obtaining the SE, as well as looking at making the measurement more applicable to physically small and electrically large enclosures, and hoping to inform future versions of shielding effectiveness standards for small enclosures. The first part of this thesis investigates the use of a comb generator as a source in an enclosure under test (EUT), which provides a statistically uniform electric field inside the EUT when combined with a small mechanical stirrer. The EUT used here is an equivalent size to a 19 inch rack unit used in many equipment rigs; therefore investigations using it are of relevance to the real world. It becomes apparent that it is important to be sure that statistical field uniformity is achieved within the EUT as well as in the test chamber. The chamber at the University of York is compared with the chamber used in Ancona, Italy. Meanwhile, it is found that the presence of a direct path or unstirred component distribution in an enclosure or chamber can change the measured SE. A study of aperture dominated EUTs reveals that it is possible to obtain an indication of the SE of an enclosure using the Q−factor. This test method has the advantage that it can be applied to enclosures that have a low SE or have many apertures, as is the case in some real enclosures. Continuing the development into testing physically small enclosures that are outside the scope of IEEE 299:1997, it is shown that a physically small enclosure can be represented by an electrically equivalent larger enclosure. This is also of use when considering IEEE 299.1.