  The electronic structures of the ground state for several different
superconducting materials, such as cuprates, conventional 3-dimensional
superconductors, doped semiconductors and low-dimensional systems, are quite
different and sometimes in contrast to what is supposed to make a
superconductor. Properties like the Fermi-surface (FS) topology,
density-of-states (DOS), stripes, electron-phonon coupling ($\lambda_{ep}$) and
spin fluctuations ($\lambda_{sf}$) are analyzed in order to find clues to what
might be important for the mechanism of superconductivity. A high DOS at $E_F$
is important for standard estimates of $\lambda's$, but it is suggested that
superconductivity can survive a low DOS if the FS is simple enough.
Superconducting fluctuations are plausible from coupling to long wave length
modes in underdoped cuprates, where short coherence length is a probable
obstacle for long-range superconductivity. Thermal disorder is recognized as a
limiting factor for large $T_C$ independently of doping.
