Nearly one half of the species of madtom catfishes was described in the single comprehensive revision of the group. Morphology is very conserved in this genus; most of the species look very similar and it is difficult to tell them apart with the usual meristic and morphometric characters used to identify species. The more conspicuously variable attributes of these secretive fishes are features of pigmentation, which also are more difficult to quantify and often vary according to substrate and water quality. These species have small to tiny or fragmented ranges, and aspects of pigmentation are important diagnostic characters of each. Over one quarter of the recognized species diversity in Noturus remains undescribed.
A number of madtom species are narrowly distributed and extremely rare, and thus are at great risk of extinction. N. trautmani may even be extinct, having been last collected in 1957.
There are currently 29 recognized species in this genus: 
^ abcdBurr, Brooks M.; Eisenhour, David J.; Grady, James M. (2005). "Two New Species of Noturus (Siluriformes: Ictaluridae) from the Tennessee River Drainage: Description, Distribution, and Conservation Status". In Armbruster, J. W. Copeia2005 (4): 783–802. doi:10.1643/0045-8511(2005)005[0783:TNSONS]2.0.CO;2.