This paper proposes a set of formal tests to address the goodness-of-fit of Markov switching models. These formal tests are constructed as tests of model consistency and of both parametric and non-parametric encompassing. The formal tests are then combined with informal tests using simulation in combination with non-parametric density and conditional mean estimation. The informal tests are shown to be useful in shedding light on the failure (or success) of the encompassing tests. Several examples are provided.