This is the third and final volume of the Keys to the Trematoda, a series on the systematics and identification of the Class Trematoda. It covers five superfamilies within the Order Plagiorchiida and the family Didymozoidae, with keys for their identification at the family, subfamily and generic levels. It also includes a key to all digenean superfamilies, including those treated in detail in volumes one and two. Previous volumes dealt with the Subclass Aspidogastrea and Order Strigeida as well as the Order Echinostomida and the first two plagiorchiidan superfamilies.