Polymers are a diverse and versatile class of materials composed of long-chain macromolecules made up of repeating structural units known as monomers. These materials display a vast array of properties that range from flexible and elastic to rigid and glassy, largely due to their structural variability across different length scales. The term structure–property relationship in polymer science refers to the correlation between the molecular, supramolecular, and macroscopic structural characteristics of a polymer and its physical, mechanical, thermal, chemical, optical, and barrier properties. The study of structure–property relationships in polymers has been a cornerstone of polymer science, driving innovation across a wide array of applications, from biomedicine to electronics and sustainable materials. By understanding the underlying principles of how molecular and macromolecular structures influence material properties, researchers can design polymers with tailored characteristics that meet the increasingly complex demands of modern industries. This chapter focused on the importance of these relationships in diverse polymer systems, X-raying in detail the molecular structures and types of polymers, the key properties of polymers, and the structure–property relationships in polymeric materials. Moreover, the challenges and future directions in structure–property engineering are discussed.