Most liquid crystals consist of molecules shaped like the rod. This chapter focuses on calamitic liquid crystals as they are the most important for applications. Calamitic and thermotropic liquid crystals are important for LCDs. Their nematic phase is the basis for both the most widely used Twisted Nematic cell with active matrix addressing, and for the Super Twist Nematic cell with passive matrix addressing. The liquid crystals used are calamitic and thermotropic in the nematic phase. The operation of this most widely applied LCD is phenomenologically described in order to give an overview over the entire flat panel display system, including the addressing scheme.