Physics motivation and detector upgrades for the new era of the ATLAS experiment

The ATLAS experiment at CERN's Large Hadron Collider will be upgraded in two stages to prepare first for the Run~3 data-taking campaign, during which the integrated luminosity will roughly double, and then for the High-Luminosity LHC program with an ultimate integrated luminosity of up to 4 ab$^{-1}$. These upgrades and high-statistics datasets will allow ATLAS to perform searches and precision measurements to constrain the Standard Model Theory (SM) in yet-unexplored phase spaces and in the Higgs sector. This contribution summarizes the ATLAS detector upgrades and selected physics prospects for Run 3 and the HL-LHC.