This book aims to provide fundamental knowledge and information for research in molecular systematics on parasitic helminths (nematode, trematode, cestode). The shreds of evidence of molecular systematics studies will be compiled and discussed in terms of the utilities and pitfalls of the genetic marker used for various purposes, which have been implemented for molecular systematics of parasitic nematodes, cestodes, and trematodes. Moreover, this book will also provide the procedure for research on molecular systematics and DNA taxonomy as the guideline to explore parasitic helminths. Finally, the further perspectives of utilizing genetic markers for molecular studies on parasitic helminths will be addressed in the context of applications from the laboratory to fieldwork such as DNA barcoding and environmental DNA metabarcoding of parasitic helminths.