Fostering is when people, often of different levels of influence, allow someone with a higher or lower level of influence than themselves to raise one of their sons (and sometimes daughters) as their own. The effect is a complex web of relations that cuts across family trees, and can mean a vast pool of people who could be chosen as an heir by tanistry. It also helps to improve relations between leading families and the families in a clachan.

It is by this process that retainers and the most powerful landowners are usually related to the Lord of the Isles, and the Lord of the Isles is related to the clan chiefs of the Highlands, but amongst the clansmen and their families it is used to give clachans a sense of unity and to ensure cooperation which makes the administrative work of the tacksman (who may himself have fostered sons from or out to the families of his clachan) easier and makes the clachan more prosperous as a result.

It also opens up new routes for handfasting.