We calculate microscopically the spectrum of a 3He impurity atom moving in 4He, determine the Fermi–Liquid interaction between 3He atoms and then calculate the pressure and concentration dependence of the effective mass and the magnetic susceptibility. The long wavelength limit of the spectrum defines the hydrodynamic effective mass. When k ≥ 1.7Å-1the 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); the relevant matrix elements are determined by the Fermi hypernetted–chain theory. 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 measured3 Landau parameter F^a_0.