They are supposed to make getting around easier.
But over-reliance on sat-navs could leave us completely lost, a study has suggested.
Scientists think our memory for places is like a mental map which we have learnt from looking at a real map of where we live.
Researchers tested 26 residents of a town, all of whom had lived there for at least two years.
They were put into a virtual-reality headset which showed a 3D model of the town at locations familiar to them, but under fog.