A variety of lines of evidence now identify the Soft Gamma Repeaters and Anomalous X-ray Pulsars as magnetars -- neutron stars whose high energy emissions are powered by the decay of an ultrastrong magnetic field. This talk will analyse some key features of SGR/AXP phenomenology -- the transient increases in the non-thermal persistent emission, the time-variable torque, and the the characteristic lifetime of 10^4 years as a bright X-ray source -- with a goal of constraining the mechanism by which the magnetic field decays and the composition of the star. The formation mechanism of magnetars is also constrained by observations of supernova explosions in other galaxies: the relative merits of black holes and magnetars as the `central engines' of hyperenergetic supernovae will be addressed.