Invariance principles provide a universal description of the behaviour of a general class of random objects. For example, if a random walk lies in the domain of attraction of a stable law, then it converges after an appropriate rescaling to the corresponding stable Lévy process. The past decades have seen rapidly growing research activity on related universal limit objects for random planar structures, such as trees or graphs embedded on a fixed surface. The talk is meant to give an introduction to this topic, outline some selected results, and discuss future research directions.