  In the most general two-Higgs-doublet model (2HDM), unitary transformations
between the two Higgs fields do not change the functional form of the
Lagrangian. All physical observables of the model must therefore be independent
of such transformations (i.e., independent of the Lagrangian basis choice for
the Higgs fields). We exhibit a set of basis-independent quantities that
determine all tree-level Higgs couplings and masses. Some examples of the
basis-independent treatment of 2HDM discrete symmetries are presented. We also
note that the ratio of the neutral Higgs field vacuum expectation values,
tan(beta), is not a meaningful parameter in general, as it is basis-dependent.
Implications for the more specialized 2HDMs (e.g., the Higgs sector of the MSSM
and the so-called Type-I and Type-II 2HDMs) are explored.
