  The Evershed flow is a systematic motion of gas that occurs in the penumbra
of all sunspots. Discovered in 1909, it still lacks a satisfactory explanation.
We know that the flow is magnetized, often supersonic, and that it shows
conspicuous fine structure on spatial scales of 0.2"-0.3", but its origin
remains unclear. The hope is that a good observational understanding of the
relation between the flow and the penumbral magnetic field will help us
determine its nature. Here I review advances in the characterization of the
Evershed flow and sunspot magnetic fields from high-resolution spectroscopic
and spectropolarimetric measurements. Using this information as input for 2D
heat transfer simulations, it has been demonstrated that hot Evershed upflows
along nearly horizontal field lines are capable of explaining one of the most
intriguing aspects of sunspots: the surplus brightness of the penumbra relative
to the umbra. They also explain the existence of penumbral filaments with dark
cores. These results support the idea that the Evershed flow is largely
responsible for the transport of energy in the penumbra.
