  The London penetration depth plays a key role in determining and uncovering
many properties of a superconductor, including homogeneity, anisotropy, isotope
effects, importance of quantum and thermal fluctuations, and facets of the
nature of superconductivity in a particular material. Guided by the generic
phase diagram in the temperature-dopant concentration plane we examine
experimental data on the temperature, isotope substitution, inhomogeneity and
magnetic field dependence of the penetration depths to uncover some facets of
the mystery of superconductivity in the cuprates
