The Mu2e experiment at Fermilab will search for the charged-lepton flavour violating
neutrino-less conversion of a negative muon into an electron in the field of an aluminum nucleus.
The Mu2e detector is composed of a tracker and an electromagnetic
calorimeter and an external veto for cosmic rays.
The calorimeter plays an important role in providing excellent particle identification capabilities,
a fast online trigger filter while aiding the track reconstruction capabilities.
The calorimeter requirements are to provide a large acceptance for ~100 MeV electrons and reach:
1) a time resolution better than 0.5 ns @ 100 MeV;
2) an energy resolution O(10\%) @ 100 MeV;
3) a position resolution of 1 cm.
The calorimeter consists of two disks, each one made of 674 undoped CsI
crystals readout by two large area $2\times3$ array of UV-extended
SiPMs of 6x6 mm$^2$ dimensions.
A large scale prototype has also been constructed and tested at the beam test facility in Frascati.
It consists of 51 pre-production crystals readout by two Mu2e SiPM.
We present the progresses done to complete the calorimeter design as well as
a summary of results obtained in the production of components and on the test beam
of the prototype.