Syntactic foams are two component materials consisting of matrix resin and hollow particles. Reinforced syntactic foams contain an additional reinforcing material. The density of syntactic foams can be tailored based on the appropriate selection of hollow particle density and volume fraction. Glass hollow particles have been a widely used filler material because their low thermal expansion coefficient provides syntactic foams with high dimensional stability. Hollow fly ash particles, called cenospheres, have also been used as filler material. Fly ash cenospheres are inexpensive and help in developing low cost syntactic foams. However, usually defects are present in the walls of cenospheres and the mechanical properties of cenosphere-filled syntactic foams are not as high as those filled with glass hollow particles at the same density level. Enhancement of mechanical properties of syntactic foams beyond those obtained by tailoring the matrix and hollow particles can be obtained by micro- and nano-scale reinforcement of the matrix material. This chapter discusses hollow particle parameters such as wall thickness and density. Structure and properties of various reinforcements including glass fibers, nanoclay, carbon nanofibers (CNFs), carbon nanotubes (CNTs), and rubber particles are also discussed.