  We provide a brief review of thermohaline physics and why it is a candidate
extra mixing mechanism during the red giant branch (RGB). We discuss how
thermohaline mixing (also called $\delta$ $\mu$ mixing) during the RGB due to
helium-3 burning, is more complicated than the operation of thermohaline mixing
in other stellar contexts (such as following accretion from a binary
companion). We try to use observations of carbon depletion in globular clusters
to help constrain the formalism and the diffusion coefficient or mixing
velocity that should be used in stellar models. We are able to match the spread
of carbon depletion for metal poor field giants but are unable to do so for
cluster giants, which may show evidence of mixing prior to even the first
dredge-up event.
