Decay Rate of a Positronium. Review of Theory and Experiment

The present status of theoretical and experimental investigations of the decay rate of a positronium is considered. The increasing interest to this problem has been caused by the disagreement of the calculated value of $\Gamma_3 (o-Ps)$ and the recent series of precise experiments. The necessity of new calculations on the basis of the quantum field methods in bound state theory is pointed out with taking into account the dependence of the interaction kernel on relative energies.