<p>The top quark is the most massive known elementary particle, and plays a pivotal role in our understanding of particle physics. Its unique properties offer valuable insights into the Standard Model and potential hints for physics beyond the Standard Model. In this thesis we present a precision measurement of the polarization of top quarks and spin correlations between top-antitop (ttbar) pairs using Run-II datasets collected from the Compact </p> <p>Muon Solenoid detector at the Large Hadron Collider. In the first part of this thesis we introduce the theoretical framework of the Standard Model and its predictions regarding top quark spin polarization and ttbar spin correlations. Next, we describe the experimental setup, reconstruction techniques, and Monte Carlo simulations used in this research. Subsequently, details of the measurement, including event selection, top quark reconstruction, and unfolding are described. The analysis achieves exceptional signal purity and precision with respect to previous measurements. The measured values of coefficients are in agreement with Standard Model expected values as well as theoretical predictions at NLO in QCD. In a first, we provide provide double-differential cross sections of top quark spin polarizations and ttbar spin correlations, as a function of the invariant mass of the ttbar system. We expect the results from the final CMS publication to supersede any results presented in the measurement part of this thesis document. In the final section of this thesis, we present a projection study of top quark spin polarization and tt ̄ spin correlations at the upcoming High Luminosity LHC. We present projections of the fraction of SM-like events and show that the upcoming detector could reduce uncertainties by as much as a factor 2. An alternative to the SM scenario is also considered in the form of Super Symmetry, and it is shown that we can significantly increase the ultimate reach of the LHC to discover top squarks in the degenerate mass corridor in the top squark-neutralino plane, or in the absence of a discovery exclude top squarks up to 600 GeV. </p>