FTLD is a purely anatomically defined concept, being unrelated to the underling cellular pathology; the sine qua non is only the presence of main lesions in the frontal and temporal lobes. FTLD, therefore, is destined to include various maladies that involve the two areas. Cases reported by Arnold Pick, "Pick's disease", are a prototype of FTLD. Because of lack of histopathological description of the brains in his reports, however, the nomenclature brought about a great confusion in its nosology; the history of establishing the concept of FTLD was that of how to seperate genuine Pick's disease. After a long chaos, the present molecular neuropathology has ultimately resolved this problem by clarifying protein constituents of neuronal and glial aggregates in FTLD. TDP-43 was first found in ALS and ALS with dementia (ALSD), and soon FUS/TLS was detected in some TDP-43-negative FTLD-U groups. At the present time, FTLD consists of three main subgroups; 1) FTLD-tau, which includes Pick disease, PSP, CBD, etc., 2) FTLD-TDP, which is subdivided into types A-D, with ALSD belonging to type B, and 3) FTLD-FUS, the members of which are aFTLD-U, NIFID, and BIBD. Further discovery of yet-undetected proteins of some FTLD-U subsets will add more subclasses.