Mixing by turbulent motions in the ocean is important for global ocean circulation, nutrient supply to surface waters, and the global ocean heat budget. Turbulent mixing takes the form of a diffusive process where the vertical flux is proportional to the concentration gradient with an eddy diffusivity coefficient, Kz, as a constant of proportionality. Osborn (1980) suggested that Kz is in turn related to the flux Richardson number (Rif) or mixing efficiency (G) that is commonly assumed to be a constant value of 0.2. However, recent observations have suggested a dependence of Rif on a variety of other parameters. Here, I discuss direct measurements of the buoyancy flux (and hence the mixing efficiency), the dependence of Rif, and examine the implications of a variable mixing efficiency on current estimates of mixing in the ocean.