Artificial materials engineered to exhibit properties that do not typically exist in nature are often called metamaterials, and these unique materials have attracted a significant amount of scientific interest in recent years. Just as conventional materials owe their properties to the average response from an ensemble of atoms and molecules, in “artificial” materials, each structural unit plays the role of an atom. The material properties are controlled not only by what elements are used and their individual properties, but also by the way they are arranged collectively in lattice-like geometrical patterns with prescribed spatial variations. This immense flexibility is one of the main reasons why material engineering has become such an explosive catalyst in so many scientific disciplines. For example, metamaterials applied to the manipulation of electromagnetic waves have demonstrated some counterintuitive concepts such as a negative index of refraction, which allows light to bend at the interface of materials in a direction opposite to what you would observe from any ordinary materials. The concept of artificial material engineering has extended beyond conventional material science and it has now started to be applied to the manipulation of heat flow.