Recent results on semileptonic decays of $b$ hadrons at LHCb
are presented, with particular emphasis on decays involving $\tau$
leptons in the final state. A new measurement of ${\cal{R}}(D^{*-})\equiv {\cal{B}}(B^0 \to D^{*-} \tau^+
\nu_{\tau})/{\cal{B}}(B^0 \to D^{*-} \mu^+\nu_{\mu})$ is
reported, by using for the first time the $\tau$ lepton decays with
three charged pions in the final state. A value of ${\cal{R}}(D^{*-}) = 0.285 \pm 0.019 \pm 0.025 \pm 0.013$ is
obtained, where the the first uncertainty is statistical, the second
systematic and the third uncertainty is due to the limited knowledge
of external branching fractions. This measurement is in
agreement with the Standard Model prediction and with previous
results.