The Tile Calorimeter (TileCal) is the hadronic calorimeter covering the central region of the ATLAS experiment. It is a sampling calorimeter with steel as absorber and scintillators as active medium. The scintillators are readout 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. Both the on-and off-detector TileCal electronics will undergo significant upgrades in preparation for the high luminosity phase of the LHC (HL-LHC) expected to begin in 2029 so that the system can cope with the HL-LHC increased radiation levels and out-of-time pileup and can meet the requirements of a 1 MHz trigger. PMT signals from every TileCal cell will be digitized and sent directly to the back-end electronics, where the signals are reconstructed, stored, and sent to the first level of trigger at a rate of 40 MHz. This improved readout architecture allows more complex trigger algorithms to be developed. The TileCal system design for the HL-LHC results from a long R&D program cross-validated by test beam studies and a demonstrator module. This module has reverse compatibility with the existing system and was inserted in ATLAS in August 2019 to test current detector conditions. The new design was tested with a beam of particles in 2021 at CERN SPS. The main features of the TileCal upgrade program and results obtained from the Demonstrator tests and test beam campaigns will be discussed.