We review recent experimental studies of smectic A layering of asymmetric mesogens with emphasis on their relationship to phase transition physics. The formation of one and two dimensional lattices with long range order in the positions of the heads (tails) of molecules is considered as having either a polar or a steric origin. The various molecular models and phenomenological descriptions reproducing the phase behaviour of particular types of layering are evaluated. The influence of molecular factors on the delicate stability of different smectic A(C) phases is discussed in detail.