Luminescence intermittency, also termed blinking, refers to spontaneous changes in the brightness of a luminescent fluorophore under continuous optical excitation. Blinking was first observed in colloidal semiconductor nanocrystals over fifteen years ago, shortly after synthetic protocols became advanced enough to produce brightly luminescent nanocrystals. The underlying physical mechanism was initially associated with long-lived photo-induced charging of the nanocrystals. In recent years, however, significant evidence has accumulated to point at a more complex physical picture of the process, which involves several distinct mechanisms and is mediated by surface charge trapping. In parallel, efforts to synthesize highly luminescent semiconductor nanocrystals that do not exhibit blinking have recently borne fruit. We review the recent progress in understanding of blinking and potential applications in bioimaging using inorganic fluorescent tags.