jats:pFerroelastic domain walls are typically twin interfaces. They contain a multitude of emerging properties, including (super-)conductivity, polarity, optically activity, and photovoltaic properties and may contain a number of magnetic properties even when there is no magnetic element in the ferroelastic matrix. Current progress to determine these properties is reviewed. Local wall properties change a little if the domain walls are deformed, but external fields can shift the location of the wall. Mobile domain boundaries carry with them the locus of any emerging property. The double flexibility of functional domain walls and the possibility to move domain boundaries provide a new design concept for device manufacturing. Some potential avenues for device designs are discussed.</jats:p>