Low-temperature photoluminescence (PL) spectroscopy is a very sensitive tool to investigate the presence of dislocations in Si. The main dislocation-related bands (D1-D4) have been attributed to a wide range of causes, either intrinsic properties of the dislocation or impurity related. PL is a competitive recombination process and the non-radiative processes need to be measured to understand the overall effect of impurities. PL spectroscopy samples a large volume in comparison to the dislocation itself and therefore gives an average effect. High-resolution room-temperature PL mapping (SiPHER) has been used to detect defects in both Si and SiGe wafers. Whole-wafer PL maps reveal the presence of slip on 300 mm Si wafers. Comparison studies with defect etching show that there is a one-to-one correlation between defects detected in the PL micro-scans and those revealed by defect etching. Whole-wafer mapping has revealed a number of different defect types in SiGe epilayers. The ability to record whole-wafer PL maps facilitates the rapid identification of inhomogeneities and defects. High-resolution PL micro-maps showed the defect area to contain a high density of misfit dislocations, and the nucleation site has strong non-radiative recombination. One common defect type was analysed using plan view transmission electron microscopy (TEM) and optical microscopy; these results revealed the presence of a high density of defect loops and stacking faults consistent with the high recombination rate at the defect site.