Reinforcing polymers is a never-ending activity allowing the use of polymers in increasingly demanding applications, where unfilled or unreinforced polymers cannot penetrate. By bringing extended mechanical performances reinforced polymers can compete with stiffer materials like metal, leading to weight reduction opportunities. Fibers are well-known polymer reinforcers, (glass fibers, carbon fibers or even high performance polymers like PEEK or PEI fibers). Latest technical advances are now coming from direct continuous compounding of fibers into the polymer matrix. Fillers are also efficient ways to reinforce polymers, improving other properties like dimensional stability and scratch resistance. Both fillers and fibers also benefit from surface treatment advances which bring increased filler/polymer (fiber/polymer) interface resistance. Latest trends foreseen are fueled by nanotechnologies (carbon nanotubes, nanoclayes, halloysite nanotubes) and naturally-based reinforcers (sisal fibers, cellulosic fibers or flour) as well as innovative ways of reinforcing polymers: by developing special compatibilisers or nucleating agents polymers are reinforced without the need to add another material into the host resin. This leads to a mono-material solution, which is a good step toward easing recycling.