Layered models are models in which the coupling constants depend in an
arbitrary way on one spatial coordinate, usually the distance from a
free surface or boundary. Here the theory of the boundary critical beh
aviour of two-dimensional layered Ising models, including the Hilhorst
-van Leeuwen model and models for aperiodic systems, is reviewed, with
a particular attention to exact results for the critical behaviour an
d the boundary order parameter.