The formalism necessary to understand the phenomenon of $\mathrm{CP}$ nonconservation as observed in the neutral-kaon system is presented. The distinction between indirect $\mathrm{CP}$ violation and direct $\mathrm{CP}$ violation is made, and the level of understanding of the phenomenon in the standard model is reviewed. Attention is placed on new experimental efforts that could definitely establish a first-order or direct effect. The authors analyze the potential for such an observation in both kaon and $B$-meson decays and give the range of predictions from the standard model. Other possible models are considered briefly.

DOI:https://doi.org/10.1103/RevModPhys.65.1113