This book leads the reader from the well-established wave description of polaritons to their particle description to quantify the polariton contribution to the heat transport along polar and metallic nanofilms, nanowires, and cavities. Over the last few decades, the surface electromagnetic waves propagating along the interface of metals (plasmon polaritons) and polar dielectrics (phonon polaritons) have been widely studied to generate and guide energy currents.  And while the optical generation of these polaritons at a given frequency is well known and exploited nowadays, their thermal excitation and propagation in a broad frequency spectrum, explored in detail in this volume, have only more recently been emerging as an effective way to enhance conductive and radiative heat currents. Written by the foremost experts and researchers in nanoscale heat transport, this book consolidates in a single comprehensive view, the findings on surface electromagnetic waves as the 4th heat carriers, besides phonons, photons, and electrons.