A colloid is a dispersion of very fine objects in a fluid [1]. These objects can be solids, liquids or gas, and the corresponding colloidal dispersion is then referred to as suspension, emulsion or foam. Colloids possess unique characteristics. For example, as their size is smaller than the wavelength of light, they scatter light. They also offer a large interfacial surface area, meaning that interfacial phenomena are of paramount importance in these dispersions. The weight of each dispersed particle being small, gravity and buoyancy forces are not sufficient to counteract the thermal random motion of the particle, named Brownian motion (in tribute to the 19th century botanist Robert Brown who first characterized it). The particles do not remain in a dispersed state indefinitely: they will sooner or later aggregate (phase separation). Thus, the colloidal state is in general metastable and colloidal stability is one of the key features to take into account when working with colloids. Among all colloids, the polymer colloid family is one of the most widely investigated [2]. Polymer colloids are used for a large number of applications, ranging from coatings, adhesives, inks, impact modifiers, drug-delivery vehicles, etc. The particles range in size from about 10 nm to 1 000 nm (1 μm) in diameter. They are usually spherical, but numerous other shapes have been observed. Polymer colloids are not uncommon in nature. For example, natural rubber latex, the secretion of the Hevea brasiliensis tree, is in fact a dispersion of polyisoprene nanoparticles in water. Synthetic polymer colloids, also called synthetic latexes, play a prominent role in industrial chemistry. Interest in synthetic latexes developed during the Second World War, when the Japanese Navy threatened access to natural Hevea, an important raw material for tire manufacturing at that time. It appeared judicious to produce synthetic polymers under the same aspect, so that downstream operations on the elastomer processing units could remain unchanged. This led to the development of the emulsion polymerization process, one of the most versatile polymerization processes [3, 4]. The words latex, polymer colloids, and dispersed polymer nanoparticles are used interchangeably for any kind of stable colloidal submicronic polymer dispersions in a solvent, which in the majority of cases is water. In this chapter we will present a few salient features of polymer colloid structure, followed by data on the synthesis of these colloids, and finally we will give several key points on colloidal stability.