  This topical review summarizes various features of magnetic penetration depth
in unconventional superconductors. Precise measurements of the penetration
depth as a function of temperature, magnetic field and crystal orientation can
provide detailed information about the pairing state. Examples are given of
unconventional pairing in hole- and electron-doped cuprates, organic and heavy
fermion superconductors. The ability to apply an external magnetic field adds a
new dimension to penetration depth measurements. We discuss how field dependent
measurements can be used to study surface Andreev bound states, nonlinear
Meissner effects, magnetic impurities, magnetic ordering, proximity effects and
vortex motion. We also discuss how penetration depth measurements as a function
of orientation can be used to explore superconductors with more than one gap
and with anisotropic gaps. Details relevant to the analysis of penetration
depth data in anisotropic samples are also discussed.
