Tests of Lepton Flavor Universality are presented using ratios of branching fractions B(B → X c τ + ν τ)/B(B → X c µ + ν µ), with X c = D * − or J/ψ , 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 3.8σ level. We also give implications and prospects of these key measurements.