Endo-N-acetyl-beta-D-glucosaminidases (ENGases) have been defined as the enzymes that hydrolyse the glycosidic bond between an N-acetyl-beta-D-glucosamine residue and the adjacent (partner) monosaccharide within an oligosaccharide chain. Three types of enzymes have been distinguished according to this definition: ENGases acting on murein (type I), those acting on chitin (type II) and, finally, those acting on N-glycans (type III). Considering that N-acetylmuramic acid is a derivative of N-acetylglucosamine (3-O-substituted by a lactyl group), only ENGases acting between two N-acetylglucosamine residues are actually known despite the fact that other possibilities of partner monosaccharides for N-acetyl-beta-D-glucosamine are reported. Similarities in the amino acid sequences were found to occur only between chitin-ENGases and N-glycan-ENGases, but the substrate specificities of these two types of enzymes are different. However, it is possible that certain enzymes are able to cleave more than one type of substrate, and this could in particular explain why the N-glycan-ENGases are largely produced by bacteria in which no potential substrate for this type of enzymes was identified. Further study in this area is expected.