Parasitic nematodes, like all aerobic organisms, require antioxidant enzymes to cope with reactive oxygen species (ROS) generated during cellular metabolism. Additionally, they have to protect themselves against ROS produced by the host. Parasitic nematode enzymes that deal with the superoxide anion radical, the superoxide dismutases (SODs), have been described in every species examined, whereas enzymes that deal with hydrogen peroxide have been difficult to identify. A major family of enzymes in mammals, the selenium-containing glutathione peroxidases (GPXs), appears to be absent, although a selenium-independent GPX family exists. These enzymes demonstrate little or no activity with hydrogen peroxide. Catalase (CAT) activity has been detected, but sequences encoding a typical CAT polypeptide have only been identified in a few species, despite the active EST sequencing projects. However, a new family of enzymes has recently been described, the peroxiredoxins (PRXs), which are abundant in parasitic nematodes and have been shown to react with hydrogen peroxide. This review summarizes the major characteristics of each of these enzyme families in general and in parasitic nematodes, emphasizing and comparing the newer data on the family of PRXs.