Extreme ultraviolet lithography (EUVL) has received significant attention for the mass production of chips at the 7 nm technology node and below. Although the lithographic resolution is constrained by the wavelength of the light source and the numerical aperture (NA) of the projection optics system, the actual resolution depends on the contrast and dose limitations. Achieving good contrast in aerial images is essential for controlling the distribution of critical dimensions across fields, wafers, and batches. One of the primary reasons for the reduced contrast of aerial images is the mask three-dimensional (M3D) effect. Unlike conventional lithography, EUVL is characterized by the M3D effect. Therefore, understanding and mitigating the M3D effect are particularly important for EUVL imaging at the resolution limit. This paper first reviews EUV masks, including their structures, fabrication processes, and defects, followed by an introduction to the M3D effect and the mitigation strategies. The technical challenges associated with high-NA EUVL are investigated. Finally, a summary and some perspectives are provided.