  We present an analysis of four sum rules, each based on chiral symmetry and
containing the difference $\rho_{\rm V}(s) - \rho_{\rm A}(s)$ of isovector
vector and axialvector spectral functions. Experimental data from tau lepton
decay and electron-positron scattering identify the spectral functions over a
limited kinematic domain. We summarize the status of the existing database.
However, a successful determination of the sum rules requires additional
content, in the form of theoretical input. We show how chiral symmetry and the
operator product expansion can be used to constrain the spectral functions in
the low energy and the high energy limits and proceed to perform a
phenomenological test of the sum rules.
