A small fraction of the synchrotron radiation from relativistic jets is expected to show circular polarization. This fraction is typically less than a couple tenths of a percent, but can be up to an order of magnitude larger in rare cases. Circular polarization is sensitive to magnetic field order along the line of sight, the lower end of the particle energy distribution, and even the matter content of jets (e+ e- vs. p e-). Unfortunately these parameters are often tied together in a complex fashion making it difficult to uniquely interpret Stokes V observations. I will discuss the use of the VLBA to study circular polarization with milli-arcsecond resolution. In addition to describing the techniques, I will review past results and present new results in the light of using high resolution observations to better constrain models of Stokes V production in relativistic jets.