Through the example of nematic liquid crystals, which are the kind of crystalline phases in which surface effects have most extensively been studied, the principal aspects of the behaviour of liquid crystals at interfaces are presented. They can be separated in two categories. First, the presence of another phase in contact with the liquid crystal induces a perturbation of the liquid crystalline order. Second, the presence of a surface limiting the liquid crystalline phase has a macroscopic effect on the bulk of the phase: in absence of any other orienting field, the surface can impose the orientation of the whole phase. This orienting effect of surfaces on liquid crystals is the so-called anchoring phenomenon. These two aspects of surface effects are linked to each other as the induced anchoring direction depends on the structure of the perturbed region.