  We present lattice results for the form factors relevant in the K -> pion and
D -> pion semileptonic decays, obtained from simulations with two flavors of
dynamical twisted-mass fermions and pion masses as light as 260 MeV. For K ->
pion decays we discuss the estimates of the main sources of systematic
uncertainties, including the quenching of the strange quark, leading to our
final result f+(0) = 0.9560 (57) (62). Combined with the latest experimental
data, our value of f+(0) implies for the CKM matrix element |Vus| the value
0.2267 (5) (20) consistent with the first-row CKM unitarity. For D -> pion
decays the application of Heavy Meson Chiral Perturbation Theory allows to
extrapolate our results for both the scalar and the vector form factors at the
physical point with quite good accuracy, obtaining a nice agreement with the
experimental data. In particular at zero-momentum transfer we obtain f+(0) =
0.64 (5).
