Lightweight materials are of great interest to transportation applications. Structural weight reduction directly translates into fuel saving and increased payload capacity. Porous materials can provide significant weight saving but their applications are limited by their low strength and modulus. This chapter provides an introduction to porous materials, which includes open- and closed-cell foams. The closed-cell foams can be further divided into foams containing gas porosity and the foams containing hollow particles. The hollow particle filled porous materials are called syntactic foams. These foams are also classified as particulate composites. Enclosure of porosity within a thin but stiff shell helps in obtaining low density in syntactic foams without a severe penalty on the mechanical properties. Syntactic foams possess superior properties under compression compared to foams comprising gas porosity in the matrix. Several micro- and nano-scale reinforcements have been used to improve the tensile and flexural strengths of syntactic foams. Nanoclay, carbon nanofibers, carbon nanotubes, glass fibers, and ceramic particles have been used as reinforcements in syntactic foams. Establishing structure-property correlations of reinforced syntactic foams will pave way to design effective lightweight composites for engineering structures. The chapter also discusses some of the present day applications of syntactic foams.