Hyaluronidases, a type of glycosidases, degrade hyaluronan and some glycosaminoglycans. These enzymes have a wide range of promising applications in the medical treatment, cosmetology, preparation of oligo-glycosaminoglycans, and so on. Many studies show that oligo-glycosaminoglycans have a lower molecular weight but a higher biological immune activity than glycosaminoglycans. In view of these important functions and applications of hyaluronidases, this review provides useful information to researchers for further theoretical studies and utilization of these enzymes. This review mainly introduces the definitions of hyaluronan and hyaluronidase, as well as the advances of classification, structures, catalytic mechanisms, enzymatic assay methods, recombinant expression, biochemical properties, and applications of hyaluronidases. This review also indicates future research directions of hyaluronidases.