Organic compounds from terrestrial and marine organisms have been used extensively in the treatment of many diseases and serve as compounds of interest both in their natural form and as templates for synthetic modifications. This chapter summarizes the present knowledge about anthelmintic effects of the extracts and some already purified natural compounds isolated from the lower marine organisms including bacteria, sponge, fungi, and algae as well as the higher plants. A brief summary on anthelmintics in use is also included to provide a background for the comparison of effective concentrations, mode of actions, and weaknesses in therapy. The main focus is placed on in vitro and in vivo activities of secondary plant metabolites (alkaloids, essential oils, flavonoids, saponins, amides, enzymes, condensed tannins, and lactones with endoperoxide bridge-artemisinins) against nematodes, trematodes, and cestodes of medical and veterinary importance, and experimental model infections. Several issues are highlighted; the synergistic effect of a number of bioactive components in plant extracts, multiple putative target sites in helminths for some of secondary plant metabolites, probably different from those of current anthelmintics, which is suggested by their modified mode of actions.