Osteoarthrosis (OA) is a major component of many osteochondrodysplastic syndromes which follow mendelian modes of inheritance. The use of molecular genetic techniques to unravel the molecular background of OA has therefore become possible. Linkage analysis, positional cloning, and candidate gene analysis have been used to identify genes which, when altered, can lead to syndromic OA. Allele association studies and sib-pair analysis have also made it possible to study the importance of a particular gene or locus in OA at the population level. Furthermore, the creation of transgenic mice and analyses of naturally occurring and chemically or radiologically induced mutations in mice are providing in vivo models for human osteochondrodysplasias and OA.