The nature of damage present in ion implanted silicon crystals has been observed by transmission electron microscopy for a wide range of ion species (A, Xe, Pd, Au, Si, Ge, at 40–100 keV) over the dose range 1013to 1017ions/cm2so that the marked dose dependence of defects after annealing (up to 900°C)and special impurity effects could be investigated. Electrical measurements (sheet resistivity and Hall effect) have been made on antimony dopedp-njunction structures irradiated with non-doping ions to support these observations and determine some electrical effects of the damage. It was found that dislocation loop structures were evolved in the case of annealed low dose irradiations where there was only a small degree of overlap of adjacent disordered zones in the as irradiated state. For irradiations which created a complete surface amorphous film, the damage remaining after 650°C anneals consisted of a compIex dislocation array with microtwins, planar defects, and often some polycrystallinity following epitaxial regrowth of the surface.