Mushy layers are liquid-filled porous media in which the liquid consists of at least two components and both heat and mass can be exchanged between the liquid and solid. Mushy layers commonly occur during the solidification of multicomponent liquids. On Earth, mushy layers occur during the solidification of magma, in sea-ice and possibly at the inner-core-outer-core boundary. Most  theoretical studies  have involved solutions of ideal mushy layer equations – equations in which the liquid and solid are treated as a single continuum and are assumed to be in perfect local thermodynamic equilibrium.  In this presentation, I will first show examples of effects of mushy layers in Earth science and then review ideal mushy layer theory. I will then describe a pore-scale mushy layer model, in which the liquid and solid are treated as separate continua, which I will use to test the approximations made in ideal mushy layer theory. An appropriately defined Peclet number will be shown to control whether ideal mushy layer theory is valid and some natural systems may be only marginally within the ideal mushy layer regime.