[en] At the Brookhaven National Laboratory a precision experiment of the muon magnetic anomaly a_μ in a magnetic storage ring is on its way. Measurements carried out on positive muons in 1999 have resulted in a_μ⁺ = 11,659,202(14)(6) x 10^-10 (1.3 ppm). A difference between this value and the theoretical value at the time of publication has lead to a careful reevaluation of all contributions. At present there are several independent theoretical values which are based on standard theory. They agree with each other and are within one to two standard deviations of the above value. The goal of the experiment is a precision of 0.35 ppm. About eight times more data are being analyzed and additional data taking is foreseen. Eventual agreement with experiment would allow to put stringent limits on parameters in theoretical approaches beyond the standard model. Disagreement could indicate new physics