Abstract An overview of the momentum and frequency dependence of effective electron-electron inter- actions which favor electronic instability to a superconducting state in the angular-momentum channel ℓ and the properties of the interactions which determine the magnitude of the temperature T$_{c}$ of the instability 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, su- perfluidity in liquid He $^{3}$, superconductivity in some of the heavy fermion compounds, in Cuprates, in pncitides and the valence skipping compound, is reviewed. To provide an anchor to the limits for T$_{c}$ , the solvable case of dilute fermions with interactions varying from very weak (BCS limit) to the unitarity scattering limit and beyond (Bose-Einstein condensation of molecules) realized in experiments on cold atoms in optical traps is also discussed. 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 for this last case 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$_{c}$ . Experiments and theoretical investigations are suggested to clarify the many unresolved issues. PACS numbers: 2