The question that is the title of this chapter is, on the one hand, an interesting and legitimate query; on the other hand, it seems strange to ask it now. The basic problem is our inability to provide a satisfactory answer. Although many responses may be proffered, none is entirely sufficient for the purposes of complete understanding. This seems odd because the popularity of the LD concept is widespread, and there is a well-defined conventional wisdom about LD in the popular culture. This popularization has resulted in both increased usage and increased vagueness that seems only to undermine the integrity of the LD concept. LD is the description of choice for a variety of symptoms surrounding poor school performance and generalleaming difficulty. Its ever-increasing generalization only serves to further weaken its usefulness. As the term has become more popular and comfortable, inappropriate presumptions about common understanding have been fostered, to the detriment of real understanding. This has created a tension between the LD academic community, which acknowledges the lack of complete understanding at least implicitly, and the professional community, which proceeds with pragmatic demands as if LD were completely understood. Thus, as the number of students identified as LD increases, our confidence in the validity of their diagnosis dwindles.