  Two classes of X-ray/$\gamma$-ray sources, the Soft Gamma Repeaters and the
Anomalous X-ray Pulsars have been identified with isolated, slowly spinning
magnetars, neutron stars whose emission draws energy from their extremely
strong magnetic field ($\sim 10^{15}-10^{16}$ G). Magnetars are believed to
form with millisecond spin period and to represent an important fraction of the
whole population of young neutron stars. Newborn magnetars can convert very
quickly their rotational energy into electromagnetic and/or gravitational
waves, by virtue of their strong magnetic fields and fast spins. This chapter
provides a brief summary of astrophysical problems and scenarios in which
millisecond magnetars are believed to play a key role: these include Gamma Ray
Bursts, Supernovae, Gravitational Wave events and Fast Radio Bursts.
