We calculate microscopically the properties of 3 He impurity atoms in 3 He-4 He mixtures, including the spectrum of a single particle and the FermiLiquid interaction between 3 He atoms. From these, we determine the pressure and concentration dependence of the effective mass and the magnetic susceptibility. The long wavelength limit of the single-particle spectrum defines the hydrodynamic effective mass. When k 1.7Å−1 the motion of the impurity is damped due to the decay into a roton and a low energy impurity mode. The calculations of the Fermi-Liquid interaction are based on correlated basis functions (CBF) perturbation theory; the relevant matrix elements are determined by the Fermi hypernetted-chain summation method. Our theoretical effective masses agree well with recent measurements [1,2] but our analysis suggests a new extrapolation to the zero-concentration limit. With that effective mass we also find a good agreement with the measured [3] Landau parameter F a 0 .