Growing awareness about eco-friendly materials and environmental regulations have encouraged researchers to use natural resources for structural applications. Natural fibers such as sisal, banana, kenaf, jute, and oil palm are abundantly available; they are cheap and possess superior mechanical properties. Sisal fiber is one of the most promising reinforcements in polymer composites because of its higher tensile strength, modulus and impact strength. Traditionally, sisal fibers were used to make ropes, fancy articles, carpets, etc. There has been growing interest in finding potential structural applications for using sisal fiber-based polymer composites, especially in automobile, marine, and aircraft structures. This chapter addresses developments in sisal fiber-based thermoset and thermoplastic composites and the effects of surface modifications through various treatments. This chapter also deals with the effects of sisal fiber hybridization with other plant and synthetic fibers on mechanical and physical properties. © 2019 Elsevier Ltd. All rights reserved.