A configuration is a colouring of the plane Z^2. Usually, studied sets of configurations are the one defined by a set of patterns which do not appear in any configuration of the set. Such sets are called subshifts. In this talk we define sets of configurations thanks to Monadic Second Order logic, and we will talk about the complexity of knowing if an MSO definable set is a subshift. Then, we will talk about the complexity of such sets.