  The orbital component of the order parameter in the cuprate high-Tc cuprate
superconductors is now well established, in large part because of phase
sensitive tests. Although it would be desirable to use such tests on other
unconventional superconductors, there are a number of favorable factors
associated with the properties of the cuprates, and a number of technical
advances, that were required for these tests to be successful. In this review I
will describe the development of phase sensitive pairing symmetry tests using
SQUID microscopy, underlining the factors favoring these experiments in the
cuprates and the technical advances that had to be made.
