Mask inspection is essential for the success of any pattern transfer lithography technology, and extreme ultraviolet lithography (EUVL), in particular, faces unique challenges. EUV masks' resonant-reflective multilayer coatings have a narrow, wavelength-specific response that dramatically affects the way that defects appear, or disappear, at various illuminating wavelengths. Furthermore, the ever-shrinking size of "critical" defects limits the potential effectiveness of deep ultraviolet inspection techniques over time. Researchers pursuing numerous ways of finding and characterizing defects on extreme ultraviolet (EUV) masks and have met with varying degrees of success. Their lessons inform the current, urgent exploration to select the most effective techniques for high-volume manufacturing. Ranging from basic research and demonstration experiments to commercial inspection tool prototypes, the authors survey the recent history of work in this area, including sixteen projects in Europe, Asia, and America. Solutions range from scanning beams to microscopy, darkfield imaging to pattern transfer. (c) 2010 American Vacuum Society. [DOI: 10.1116/1.3498757]