Advances in genetics have probably had more practical applications in the field of the inherited neurological disorders than in any other area of medicine. A remarkably high number of serious neurological disorders follow single-gene Mendelian inheritance, in both childhood and adult life. Genetic testing now forms an important and integral part of both neurological and clinical genetics practice and has greatly enhanced both diagnostic precision and the options associated with genetic counseling. This chapter focuses on the use and implications of testing, especially in relation to genetic counseling. Risk estimations in relation to carrier testing for both autosomal and X-linked recessive disorders may, in fact, be quite complex and may need the overall family structure to be considered. The chapter also presents some of the numerous issues that need to be considered in relation to presymptomatic testing.