This chapter examines the following concepts: polymers, plastics and resins defined, polymerization, naming, characteristics, and molecular weight, solidification, melting, and other thermal considerations, thermoplastics and thermosets, aromatic and aliphatic materials, matrix-dominated properties, wet-out of fibers, additives: fillers, pigments, viscosity control agents, surface agents. Matrix materials can be polymeric, metallic, or ceramic. This chapter discusses the general nature and properties of polymeric matrices. The process of linking the small units (monomers) into long chains (polymers) is called polymerization. Polymerization can proceed by several different mechanisms. The two most common are addition polymerization (also called free-radical polymerization and chain-growth polymerization) and condensation polymerization (also called step-wise polymerization). Thermoplastics are resins that are solids at room temperature. Thermosets are resins that are usually liquids (or easily melted solids) at room temperature.