  The tunneling between two parallel two-dimensional electron gases has been
investigated as a function of temperature $T$, carrier density $n$, and the
applied perpendicular magnetic field $B$. In zero magnetic field the
equilibrium resonant lineshape is Lorentzian, reflecting the Lorentzian form of
the spectral functions within each layer. From the width of the tunneling
resonance the lifetime of the electrons within a 2DEG has been measured as a
function of $n$ and $T$, giving information about the density dependence of the
electron-impurity scattering and the temperature dependence of the
electron-electron scattering. In a magnetic field there is a general
suppression of equilibrium tunneling for fields above $B=0.6$ T. A gap in the
tunneling density of states has been measured over a wide range of magnetic
fields and filling factors, and various theoretical predictions have been
examined. In a strong magnetic field, when there is only one partially filled
Landau level in each layer, the temperature dependence of the conductance
characteristics has been modeled with a double-Gaussian spectral density.
