The accidents that occurred at the Three Mile Island (TMI) Unit 2 plant in Pennsylvania in March 1979 and the Fukushima Dai-Ichi plants in Japan in March 2011 are generally and correctly regarded as two of the three most serious in the history of commercial nuclear power (Chernobyl, of course, is the third). For that reason, both accidents need to be carefully studied and appropriate lessons need to be learned.  Although TMI and Fukushima differed in causes and consequences, they provide powerful incentives to investigate what happened and to draw conclusions that, if properly applied, can help to prevent, or at least mitigate the effects of, nuclear power accidents in the future.