The positron annihilation technique can provide a sensitive measure of defect density in metals. In this program the technique has been used to monitor defects generated during plastic deformation by cold work or fatigue cycling. The primary goals have been 1) to assess the degree of sensitivity of the technique, 2) to correlate positron annihilation readings with observed microstructural changes to better understand the physical basis for these readings, and 3) to determine correlations between positron annihilation measurements and number of fatigue cycles. Examination of fatigued samples by transmission electron microscopy indicates some correlation between dislocation density and positron annihilation lineshape parameter (determined by the Doppler broadening technique). However, annealing studies of deformed samples indicate that positron annihilation response in 316 stainless steel is sensitive primarily to excess vacancies generated during the deformation and is less sensitive to dislocation density. Data on deformed nickel show sensitivity to both vacancies and dislocations. In general, lineshape parameter values tend to achieve a constant level at approximately 10 per cent of fatigue life.