  In the last few years, evidences for a long-lived and sustained engine in
Gamma Ray Bursts (GRBs) have increased the attention to the so called
millisecond-magnetar model, as a competitive alternative to the standard
collapsar scenario. I will review here the key aspects of the {\it millisecond
magnetar} model for Long Duration Gamma Ray Bursts (LGRBs). I will briefly
describe what constraints, present observations put on any engine model, both
in term of energetic, outflow properties, and the relation with the associated
Supernova (SN). For each of these I will show how the millisecond magnetar
model satisfies the requirements, what are the limits of the model, how can it
be further tested, and what observations might be used to discriminate against
it. I will also discuss numerical results that show the importance of the
confinement by the progenitor star in explaining the formation of a collimated
outflow, how a detailed model for the evolution of the central engine can be
built, and show that a wide variety of explosive events can be explained by
different magnetar parameters. I will conclude with a suggestion that magnetars
might be at the origin of the Extended Emission (EE) observed in a significant
fraction of Short GRBs.
