In 1991 the population census presented data on annual migration of Gaelic speakers for the first time, and again in greater detail in 2001. Flows of Gaelic- and non-Gaelic-speaking populations can thus be assessed and analysed. These movements between relevant areas for Gaelic diasporise Gaelic speakers from 'heartland' areas into lowland and urban Scotland. In 1991 40% lived in such areas, increasing to 44% in 2001. This process is gaining momentum, with mobility of Gaelic speakers matching the majority population at over 11% annually. By 2011 almost half of all Gaelic speakers were living outwith the traditional Highlands and Hebrides area. Non-Gaelic speakers increasingly move to the 'Gaelic' areas and weaken the incidence of Gaelic locally. The increasing rate of marriage and family-formation between Gaelic and non-Gaelic partners weakens Gaelic in intergenerational transmission. In 1991 only one Gaelic speaker in three lived in an all-Gaelic-speaking household. Within ten years this proportion had weakened to one in five. A model of the causes of Gaelic-to-English language-shift can be developed. Increasing geographical mobility and diminished life-chances cause Gaelic speakers to leave their home areas. Non-Gaelic-speakers are drawn in. The majority of Gaelic speakers come to live as a minority amongst non-Gaelic speaking populations elsewhere. Young Gaelic speakers increasingly meet and form families with non-Gaelic-speaking partners. Without powerful Gaelic institutions outwith the family, English predominates in family life, and Gaelic ceases to be transmitted. This can successfully be countered by the development of Gaelic-medium education, Gaelic-language media, and Gaelic social institutions. The results of such language-planning by Bòrd na Gàidhlig, MG Alba, and the education system can be illustrated demographically from census data, and some measure of success can be demonstrated.