Filarial nematodes have been known to cause human disease for many centuries. Lymphatic filariasis is a common disease in the developing part of the world and much has been written about diagnosis and treatment of this scourge. Wuchereria , Brugia and Onchocerca (especially O. volvulus) have a wide pattern of distribution with severe morbidity. Given the years of scientific work in this field, many drugs that work against these parasites are available today and are attempting to control these infections. In this chapter, the focus is on those filarial nematodes that do not have humans as their primary host. Instead, the filarial organisms that usually parasitize other animals and cause human infection due to a variety of factors are discussed. These factors include: 1. Proximity of humans to the primary host, 2. Proximity of humans to the vector, 3. Changing ecology with introduction of different animals (both host and vector) into new environments, 4. Increasing human mobility, 5. Special scenarios concerning humans, including altered immune function (immunosuppressed due to drugs, auto-immune illness, immunosuppressive diseases), There has been a recent interest in this field because newer diagnostic techniques, including polymerase chain reaction (PCR) assays, DNA primers and electron microscopy have become widespread in use. This will eventually enhance our understanding of the pathophysiology of infections with these seemingly rare filarial organisms.Much of the early work in this field was done in a few specialized centers. As information about these parasites (through the worldwide web) and diagnostic techniques are now widely available, it is our hope that more work regarding these nematodes will be carried out in the developing countries where these infections are common. In this chapter, we focus on Dirofi laria, Meningonema, Loaina, Dipetalonema and certain species of Onchocerca and Brugia.