Direct, in situ observations of the electrons and Langmuir waves (plasma oscillations) associated with interplanetary Type III bursts are among the most important pieces of evidence for the plasma emission theory of solar radio bursts. Type III bursts exhibit moderate levels of circular polarization, typically less than 50%. This is lower than expected from plasma emission and is likely due to depolarization from scattering by density inhomogeneities and other propagation effects.