This chapter explores cast iron's structure, classification, and applications in the automotive industry. Cast iron forms microstructures based on graphite inclusion types, influencing its mechanical properties. Major types include grey, white, ductile, compacted graphite, vermicular graphite, and austempered ductile iron. Grey cast iron is notable for its wear resistance and thermal conductivity, which makes it ideal for engine blocks and brake components. White iron, rich in cementite, is hard and wear-resistant. Ductile and austempered ductile irons offer high strength and ductility and are used in crankshafts, suspension, and chassis parts. Compacted and vermicular graphite irons balance strength and thermal management and are suitable for high-performance engines. Mechanical properties such as tensile strength, ductility, and fatigue resistance vary by composition and treatment. Cast iron remains essential in automotive engineering due to its cost-effectiveness, damping capacity, and machinability, with innovations continuing to expand its application.