  An overview of the momentum and frequency dependence of effective
electron-electron interactions which favor electronic instability to a
superconducting state in the angular-momentum channel $\ell$ and the properties
of the interactions which determine $T_c$ is provided. Both interactions
induced through exchange of phonons as well as purely electronic fluctuations
of spin density, charge density or current density are considered. Special
attention is paid to the role of quantum critical fluctuations including
pairing due to their virtual exchange as well as de-pairing due to inelastic
scattering. In light of the above, empirical data and theory specific to phonon
induced superconductivity, in cold atoms, superfluidity in liquid $He^3$,
superconductivity in some of the heavy fermion compounds, in Cuprates, in
pncitides and the valence skipping compound, is reviewed. The physical basis
for the following observation is provided: The universal ratio of s-wave $T_c$
to Fermi-energy for fermions at the unitarity limit with attractive
interactions is about 0.15, the ratio of the maximum $T_c$ to the typical
phonon frequency in phonon induced s-wave superconductivity is of the same
order; the ratio of p-wave $T_c$ to the renormalized Fermi-energy in liquid
$He^3$, a very strongly correlated Fermi-liquid near its melting pressure, is
only $O(10^{-3})$; in the Cuprates and the heavy-fermions where d-wave
superconductivity occurs in a region governed by a special class of
quantum-critical fluctuations, this ratio rises to $O(10^{-2})$. These
discussions also suggest factors important for obtaining higher $T_c$.
