  The superconducting materials that make up an MKID have a significant effect
on its performance. The $T_\textrm{c}$ and normal state resistivity
$\rho_\textrm{N}$ of the film determine the penetration depth $\lambda$ and
therefore how much kinetic inductance it has. The ratio of kinetic inductance
to total inductance ($\alpha$), the volume of the inductor, and $Q_\textrm{m}$
determines the magnitude of the response to incoming energy. The quasiparticle
lifetime $\tau_\textrm{qp}$ is the characteristic time during which the MKID's
surface impedance is modified by the incoming energy. Many materials have been
explored for use in superconducting resonators and MKIDs, but that information
is often not published or scattered around the literature. This chapter
contains information and references on the work that has been done with thin
film lithographed circuits for MKIDs over the last two decades. Note that
measured material properties such as the internal loss quality factor
$Q_\textrm{i}$ and quasiparticle lifetime $\tau_\textrm{qp}$ vary significantly
depending on how the MKID superconducting thin film is made and the system they
are measured in, so it is best to interpret all stated values as typical but
not definitive. Values are omitted in cases when there aren't enough
measurements or there is too much disagreement in the literature to estimate a
typical value. In order to be as complete as possible some unpublished results
from the author's lab are included and can be identified by the lack of a
reference. Unless noted all films are polycrystalline or amorphous.
