Most of the industrial plastics used up to now consist of flexible macromolecules in which the repeat units are joined together in a nonlinear fashion by rotating bonds. Stiff-chain polymers, which in the ideal case have a rodlike shape, have received much less attention. The reason for this is that such polymers usually exhibit an exceedingly low solubility. Very often, they are absolutely insoluble and do not melt. However, in recent years these polymers have aroused technical interest because high-modulus fibers and moldings can be fabricated from them. These favorable mechanical properties are directly related to the nearly parallel arrangement of the rodlike macromolecules in the solid state which is achieved through spinning or extrusion from a liquid crystalline (nematic) state. It is therefore evident that a thorough understanding of the phase behavior and the structure is required for a universal technical utilization of these materials. The great variety of methods used in polymer science today has led to a deeper understanding of stiff-chain polymers. Experimental results together with theoretical modeling of the phase behavior have direct implications for the practical use of these macromolecular materials.