Gluon- and quark-jet multiplicities with NNNLO and NNLL accuracy.

We present a new approach to consider and include both the perturbative and the nonperturbative contributions to the multiplicities of gluon and quark jets. Thanks to this new method, we have included for the first time new contributions to these quantities obtaining next-to-next-to-leading-logarithmic resummed formulas. Our analytic expressions depend on two nonperturbative parameters with a clear and simple physical interpretation. A global fit of these two quantities shows how our results solve a long-standing discrepancy in the theoretical description of the data.