C9ORF72 and the 43 kDa TAR DNA-binding protein (TDP-43) are key molecules in the development of TDP-43 pathology in amyotrophic lateral sclerosis (ALS). The hexanucleotide repeat expansion in C9ORF72 also leads to frontotemporal lobar degeneration, whereas mutation of TARDBP mainly causes ALS, indicating that TDP-43 plays a more fundamental role in the development of ALS. In tissues affected with ALS, TDP-43 is dislocated from the nucleus, forms cytoplasmic inclusions, and is phosphorylated and truncated. Accumulating evidence suggests that the disappearance of TDP-43 from the nucleus precedes inclusion formation, indicating that its disappearance from the nucleus is crucial in the development of TDP-43 pathology. Alterations in the quality and quantity of TDP-43 might result in the disappearance of TDP-43 from the nucleus. Regarding quality, phosphorylation and truncation of TDP-43 is not necessary for its disappearance from the nucleus or for inclusion formation. Although it has been speculated that studies of TDP-43 harboring ALS-associated mutations are useful for understanding the molecular pathogenesis of sporadic ALS, the functional and biochemical differences between mutated and wild-type TDP-43 remain unclear. Regarding quantity, an increased amount of TDP-43 is an attractive hypothesis as it has been shown that increased amounts of TDP-43 are toxic. Moreover, several reports have suggested that increased levels of TDP-43 are found in sporadic ALS as well as in ALS with TDP-43 mutations. However, these findings remain controversial. Increased understanding of the mechanisms responsible for regulating TDP-43 will provide a basis for determining the molecular pathogenesis of ALS.