Family studies have demonstrated the genetic etiology of concomitant strabismus (prevalence in different populations 1-5%, positive family history in 37% on average), as have twin studies (mean concordance in monozygous twins 73% compared to 35% in dizygous twins). In Duane's syndrome, three chromosomal loci have been identified to date: 2q31, 8q13, and 22q11; loci have also been identified in Moebius syndrome (various inheritance patterns): 13q12.2-q13, 3q21-q22, and 10q21.3-q22, as well as in congenital fibrosis of the extraocular muscles (CFEOM; 3 loci, 1 gene).(1) Already identified are the genes for a number of rarer muscular dystrophy syndromes with ocular involvement, various forms of congenital myasthenia, and mitochondrial diseases (with the primary defect located either in the mtDNA, resulting in a mitochondrial inheritance pattern, or in the nuclear DNA with Mendelian inheritance). A number of hereditary retinal diseases (Mendelian inheritance) may also be associated with strabismus. More than one gene is likely to be involved in the frequently occurring concomitant strabismus, making the analysis more difficult. Patients with chromosomal rearrangements, large families and isolated populations (see also the contributions by Preising and Zitzlsperger et al. in this issue)(2,3) will be instrumental in gene identification. In view of the high prevalence of concomitant strabismus, the disclosure of its etiology will have considerable medical, psychosocial and health-cost impact.