Crystal defects, such as dislocations and twin/stacking faults, significantly influence the mechanical and functional properties of materials. Controlled adjustement of type and density of lattice defects is referred to as crystal defect engineering. Severe plastic deformation (SPD) and subsequent annealing are effective methods for tailoring the defect structure in metallic materials. In this presentation, the recent results on engineering of lattice defects in multi-principal element alloys (MPEAs) are overviewed. MPEAs contain at least three constituents with similar fractions. High-entropy alloys (HEAs) are also included in the class of MPEAs. These materials exhibit superior properties, such as high strength, compared to classical metals and alloys. Crystal defect engineering can further improve the behavior of MPEAs. The type and density of defects in MPEAs were tailored by SPD and subsequent annealing. The defect structure was characterized by direct microscopic methods and the non-destructive X-ray line profile analysis (XLPA). The latter technique studies a much higher volume than electron microscopy, therefore XLPA describes the lattice defects with a higher reliability. The correlation between the defect structure and the properties of SPD-processed MPEAs is revealed and discussed in detail.