The neutral kaon system at a $\phi$-factory offers a unique possibility to perform fundamental tests of $CPT$ invariance, as well as of the basic principles of quantum mechanics. The most recent and significant limits on $CPT$ violation are reviewed, including the ones related to possible decoherence mechanisms or Lorentz symmetry breaking, which might be induced in a quantum gravity scenario. The present experimental results show no deviations from the expectations of quantum mechanics and $CPT$ symmetry, while the accuracy in some cases reaches the interesting Planck scale region. The KLOE-2 experiment, presently taking data at the DA$\Phi$NE facility, will further improve these results.