In conventional smectic liquid crystals (LC) the transition into the tilted Sm C phase is accompanied by a decrease of smectic layerspacing. This is a very negative factor for electro-optic displays based on ferro- and antiferroelectric smectic LC because the layer compression leads to a buckling of layers in a chevron geometry. This results in a serious degrade of the quality of such devices. Recently a number of novel smectic materials have been discovered with practically no layer compression. These materials attract a significant interest from industry and academia; they seem to represent a novel type of tilting transition which may also be observed in other soft matter systems including organic monolayers, artificial and biological membranes. A novel molecular theory of SmA - Sm C transition will be developed which explains why some materials do not show a layer contraction, and why others do, and how the molecular structure can affect the nature of the tilting transition. The theory will explain the existing experimental data including the role of low orientational order and weak interlayer correlations. Molecular models for smectics without layer compression will be proposed, based on model interaction potentials, and molecular parameters will be identified which enable one to distinguish between conventional and novel smectic materials.