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The aetiology of hemifacial microsomia is uncertain vascular, metabolic, teratogenic and genetic factors have been proposed as aetiological influences. The evidence for a genetic basis comes from observations of the phenotype in individuals carrying chromosomal rearrangements, and from a small number of studies involving families in whom the condition appeared to be transmitted in a Mendelian fashion. Malformations reminiscent of hemifacial microsomia have also been seen in knockout and mutant animal models, and these have given rise to a number of candidate genes for the disease. Two main objectives were defined at the start of this project. The first of these was to expand upon the currently available clinical data by interviewing and examining patients with hemifacial microsomia or isolated microtia who presented to two specialist treatment centres in London, Great Ormond Street Hospital and Mount Vernon Hospital. Alongside this a genome scan was performed on a family in which hemifacial microsomia appeared to be segregating in an autosomal dominant manner. Whilst definite linkage to a single locus could not be inferred from the results of the genome scan, a few regions of interest were identified, and linkage to a large proportion of the genome was excluded. One area of particular interest, 2q32 - 2q37, was analysed in greater detail, and other regions together with their potential disease genes have been highlighted.