  Millisecond pulsars in tight binaries have recently opened new challenges in
our understanding of physical processes governing the evolution of binaries and
the interaction between astrophysical plasma and electromagnetic fields.
Transitional systems that showed changes from rotation-powered to accretion
powered states and vice versa have bridged the populations of radio and
accreting millisecond pulsars, eventually demonstrating the tight evolutionary
link envisaged by the recycling scenario. A decade of discoveries and
theoretical efforts have just grasped the complex phenomenology of transitional
millisecond pulsars from the radio to the gamma-ray band. This review
summarises the main properties of the three transitional millisecond pulsars
discovered so far, as well as of candidates and related systems, discussing the
various models proposed to cope with the multifaceted behaviour.
