In the previous volume, we recalled the existence of a wide variety of smectic phases, the simplest example being the smectic A. This phase is only distinguished from the uniaxial nematic phase by the presence of positional order along the direction of average molecular alignment (still denoted by the director n). This lamellar structuring confers remarkable properties upon the smectic A phase, the most apparent being the elastic behavior exhibited when the layers are compressed or dilated. This novel property is however not the only one, as we shall see throughout this chapter, mostly dedicated to the smectic A-nematic phase transition. It will be shown that, despite its apparent simplicity, this transition poses a multitude of questions, some of them still waiting for a satisfying solution.