Parasitism, especially gastrointestinal (GI) nematodes is one of the major problems in sheep throughout the world. The economic loss to the farmers and the industry is substantial in terms of reduced productivity, animal mortality and cost of anthelmintic treatments. Furthermore, the evolution of parasite species resistant to a wide range of currently available anthelmintics has become a major concern for the sheep industry that necessitates the search for alternatives in parasite control measures. The approaches currently under investigation for parasite control, other than anthelmintic treatment are; vaccines against nematodes, biological control (nematophagus fungi), breeding for nematode resistance or combination of some or all of them. Breeding for nematode resistance might be one of the economically viable and technically feasible options, because many researchers have already shown that there exists considerable variation within breeds resistant to a wide range of GI nematode species. The genetic parameters of nematode resistance traits and response to selection by incorporating resistance trait in selection index in Texel sheep breed have been discussed in this book.