The Tile Calorimeter (TileCal) is the hadronic calorimeter covering the central region of the ATLAS experiment. It is a scintillating plastic tiles and steel absorbers sampling calorimeter. The scintillators are read-out by wavelength shifting fibers coupled to photomultiplier tubes (PMTs). The TileCal response and its readout electronics are monitored to better than 1% using radioactive source, laser and charge injection systems. The TileCal electronics will undergo major upgrades for the high luminosity phase of the LHC (HL-LHC), so that the system can cope with increased radiation and can meet the requirements of a 1-MHz trigger. Digitized signals from all PMTs are sent to the back-end electronics and to the first level of trigger at 40 MHz. This will provide better precision in the trigger system and will allow the development of more complex trigger algorithms. The TileCal upgrade program has included extensive R&D and test beam studies. A Demonstrator module was inserted in ATLAS in 2019 for testing in actual detector conditions. The main features of the TileCal upgrade program and results obtained from the Demonstrator tests are discussed.