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We present lattice results for the form factors relevant in the K -> pion andD -> pion semileptonic decays, obtained from simulations with two flavors ofdynamical twisted-mass fermions and pion masses as light as 260 MeV. For K ->pion decays we discuss the estimates of the main sources of systematicuncertainties, including the quenching of the strange quark, leading to ourfinal result f+(0) = 0.9560 (57) (62). Combined with the latest experimentaldata, our value of f+(0) implies for the CKM matrix element |Vus| the value0.2267 (5) (20) consistent with the first-row CKM unitarity. For D -> piondecays the application of Heavy Meson Chiral Perturbation Theory allows toextrapolate our results for both the scalar and the vector form factors at thephysical point with quite good accuracy, obtaining a nice agreement with theexperimental data. In particular at zero-momentum transfer we obtain f+(0) =0.64 (5).

Vector and scalar form factors for K- and D-meson semileptonic decays from twisted mass fermions with Nf = 2