Stith Thompson (March 7, 1885 – January 13, 1976) was an American scholar of folklore. He is the "Thompson" of the Aarne-Thompson classification system, which indexes certain folktales by their structure and assigns them AT numbers. He also developed an alpha-decimal motif-index system (A~Z followed by numeral) for cataloging individual motifs.
It is a good classification, but I find it a bit too dry and difficult to navigate. In my studies, I prefer Stith Thompson's The Folktale. It is much more readable and includes indexes for both Aarne-Thompson classification, as well as his own (which I find much more helpful anyway).