Neodymium trisborohydride, associated with MgR2 alkylating agents, affords an efficient initiator for the stereospecific polymerization of isoprene. This system presents good activity (150 kg of polyisoprene/mol of Nd/h; 95% yield in 2 h) and is trans-stereospecific (up to 97.7%), and its polymerization shows a quasi-living character. The molecular weights are controlled by the [Isoprene]/[Nd] ratio, with a polydispersity index lower than 1.6. A full study is presented here, including the influence of the nature of the alkylating reagent (MgR2), the solvent, the reaction temperature, the [MgR2]/[Nd] ratio, and the replacement of neodymium by other lanthanides. This work also includes a kinetic study of the system and a detailed NMR study of the microstructure of the trans-polyisoprene obtained, providing mechanistic insights about the polymerization reaction.