This course deals with a system of theoretically derived inherent laws of structures and properties of yarns. Each module of this course is started with definitions, terminologies and fundamental relations. Then, the theoretical models are presented namely from the initial assumptions through the mathematical derivation to the final relations. Such theoretical results are mostly compared with the experimental ones. The topics in this course include basic characteristics of fibers and yarns; relation among yarn count, twist, and diameter; helical model of fibers in yarns; mass irregularity of yarns; radial fiber migration in yarns; yarn shrinkage due to washing; and tensile mechanics of yarns.