Tests of Lepton Flavor Universality are presented using ratios of branching fractions $\BR(B \to X_c \tau^+ \nu_{\tau})/\BR(B \to X_c \mu^+\nu_{\mu})$, with $X_c=D^{*-}$ or $\jpsi$, using the data collected by the LHCb experiment during 2011 and 2012. When combined with the results from the BaBar and Belle experiments, the Standard Model is challenged at the $3.8\sigma$ level.
We also give implications and prospects of these key measurements.