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The conditions under which submicroscopic defects can be revealed due to decoration by the precipitation of vacant lattice sites from a supersaturated solution have been investigated in aluminium. Two particular defects , rows of loops along<110> directions end narrow faulted dipoles with their axes along the same direction, were studied in specimens quenched to and deformed at low temperatures and subsequently aged up to room temperature. The primary observations an! that the visibility of these defects is favoured by high purity end a moderate quenching temperature, i.e. by conditions where a moderate supersaturation of vacancies is present after quenching, without the existence of many nuclei for the formation of self-sinks. Thus the conditions under which the faulted dipoles and rows of loops are observed are just those where a decoration phenomenon would be expected. The decoration technique has been used in quenched and deformed single crystals oriented for single slip to investigate the mechanism of formation of these defects. It is suggested that the observed narrow dipoles along <110> grew from similar submicroscopic dipoles produced from sessile jogs on moving screw dislocations. It is then proposed that the observed rows of loops along are formed in two ways: from the break-up of these submicroscopic dipoles at an early stage, and/or from the break-up of vacancy platelets formed .long <110> by the same mechanism. This hypothesis is confirmed by the single crystal experiments which show that the faulted dipoles lie along a particular <110> direction. A 90º jog in a screw dislocation can produce a faulted dipole along either of the <110> directions in the slip plane not parallel to its Burgers vector. The observed dipole direction, for a given tensile axis, was always found to coincide with that predicted from the known stress on the jog. The occurrence of a high density of helical dislocations is reported in zone-refined aluminium quenched to temperatures above room temperature and deformation during the quench. The conditions of quenching and deformation indicate that helices are not normally seen in pure metals partly due to the presence of effective competing sinks and partly due to the annihilation of screw dislocations by cross-slip during plastic deformation. The helices observed in aluminium are not circular helices tut consist of straight segments. It is shown that these helices are approximately the geodesics on a right prism with a rhombus base, whose generator is the Burgers vector b and whose faces are the (111) planes containing b. This configuration is attributed to the effect of dislocation core energy, the core energy being lowest if the dislocation lies in a (111) plane containing b. i.e. a glide plane. This observation is used to estimate the ratio of the core energy of a dislocation when it is not on its glide plane and when it is on it. Finally, a sensitive technique for the detection of radiation damage. , by the annihilation of small quenched-in vacancy loops during irradiation, is described. An increase in certain inelastic scattering processes is known to occur under anomalous absorption conditions due to higher electron density at the atom positions. Using the detection technique mentioned above. a similar expected enhancement in displacement damage during anomalous absorption was investigated in the 200 kV electron microscope. Preliminary results suggest that this effect, if present, is small.