Cellulases, as environmentally appropriate catalysts specifically acting on cellulosic substrates, are important for the industrial conversion of lignocellulose and modification of cellulose products. After decades of research, a fundamental understanding of cellulase-mediated hydrolysis of cellulose is that its ability to processively act as a key for the complete enzymatic hydrolysis of natural crystalline cellulose. Two types of processive cellulases are known: exoglucanases and processive endoglucanases. Exoglucanases are typical processive enzymes, and they have been studied in detail so that their modes of action and mechanisms are reasonably well characterized. Conversely, endoglucanases are less well characterized because of the non-universality and structural diversity. However, processive endoglucanases have certain characteristics that exoglucanases lack such as hydrolysis product diversity and independent hydrolyze natural crystalline cellulose. Therefore, besides the conversion of cellulose to monosaccharide, they might be useful for modification of fibrous substrates and preparation of cellulose oligosaccharides. Herein, we review in detail the sources, hydrolysis products, application, and possible hydrolysis mechanisms of processive endoglucanases.