  The effect of proximity to a Mott insulating phase on the charge transport
properties of a superconductor is determined. An action describing the low
energy physics is formulated and different scenarios for the approach to the
Mott phase are distinguished by different variation with doping of the
parameters in the action. A crucial issue is found to be the doping dependence
of the quasiparticle charge which is defined here and which controls the
temperature and field dependence of the electromagnetic response functions.
Presently available data on high-T$_{c}$ superconductors are analysed. The
data, while neither complete nor entirely consistent, suggest that neither the
quasiparticle velocity nor the quasiparticle charge vanish as the Mott phase is
approached, in contradiction to the predictions of several widely studied
theories of lightly doped Mott insulators. Implications of the results for the
structure of vortices in high-T$_{c}$ superconductors are determined. The
numerical coefficients in the field-dependent specific heat are given for
square and triangular vortex lattices.
