The heat flux to the target plates qp is a crucial design parameter for fusion machines. In attached regimes the majority of the power entering the SOL across the separatrix flows to a narrow region on the divertor target plates. Predictions of heat loads in future machines are typically based on scaling laws derived from experimental data. qp can be measured by infrared (IR) cameras viewing the target plates. Alternatively, Langmuir probes can be used to determine qp from the temperature and particle flux measured at the sheath edge in close proximity to the target plates (<<1mm). Measuring qp with probes involves physics of the sheath in which energy is transferred from electrons to ions. Relating qp to the electron temperature Te and the particle flux G at the sheath edge leads to the definition of the sheath heat transmission coefficient g = qp = TeG. g can be expressed in terms of plasma parameters.