The high resolution of the SEM is especially useful for qualitative and quantitative applications for both nanotechnology and nanomanufacturing. But, should users be concerned about the imaging and measurements made with this instrument? Perhaps one should or, at a minimum, understand some of the uncertainties associated with those measurements. It is likely that one of the first questions asked when the first scanning electron micrograph was ever taken was: “...how big is that?” The quality of that answer has improved a great deal over the past few years, especially since SEMs are being used as a primary tool on semiconductor processing lines to monitor the manufacturing processes. These needs prompted a rapid evolution of the instrument and its capabilities. Over the past 20 years or so, instrument manufacturers, through this substantial semiconductor industry investment of research and development (R&D) money, have vastly improved the performance of these instruments. All users have benefitted from this investment, especially where metrology with an SEM is concerned. But, how good are these data? This presentation will discuss a sub-set of the most important aspects and larger issues associated with imaging and metrology with the SEM. Every user should know, and understand these issues before any critical quantitative work is attempted.