Publisher Summary This chapter discusses some important aspects of surface-enhanced Raman scattering (SERS) substrates in general, with a strong emphasis on metallic colloids in particular. That serves as an example to discuss more general aspects of SERS such as the characterization of SERS substrates, or the importance of molecular adsorption. SERS substrates can be tentatively classified into three main classes: Metallic particles (usually nano-particles) in solution, such as colloidal solutions; Planar metallic structures, such as arrays of metallic nano-particles supported on a planar substrate; and Metallic electrodes. Metallic Ag and Au colloids are typically produced by a reduction reaction in solution through several possible chemical routes. The colloids exist in solution (often in water for SERS applications) only because they are stabilized by Coulombic (or sometimes steric) repulsions among particles. This normally requires the presence of a stabilizing agent, which coats the surface of the colloids and prevents them from aggregating. In some cases, a single chemical compound plays both the role of reducing agent and stabilizer; this is the case of sodium-citrate-reduced colloids, one of the most commonly used types in SERS (also called Lee-&-Meisel colloids). This chapter further discusses the problem of the stability of colloids. In addition, it also discusses some basic aspects of colloid science that are particularly relevant to SERS experiments, in particular the problem of colloid stability.