Yarns are an important basic element for both the production and the assembly of textile reinforcement structures. They consist of either a 100 % reinforcement fibers or a blend of reinforcement and matrix fibers. They are made from filaments and/or staple fibers by means of different technologies, which allow the customization of structure and properties of the yarns according to the respective functional requirements. These chapters give an overview of the yarns currently used in lightweight construction and shows that their design has considerably influence of further processing and on the characteristics of composite materials. During textile processing, the yarns have to be processed easily at high speeds and must be formable force- or form-fit. Defined, anisotropic characteristics are achieved by the preferred orientation of the fibers in the yarn and the 2D or 3D yarn orientation during the production of textile semi-finished products. In the composite material itself, the yarn structures offer mechanic fixations.