The rotational spectrum of diethyl amine as observed by molecular beam Fourier transform microwave spectroscopy shows splittings due to nitrogen inversion, (14)N quadrupole coupling, and internal rotation of the methyl groups. The rotational constants of diethyl amine were determined to be A = 17.61499170(11) GHz, B = 2.103650248(49) GHz, and C = 1.981332501(47) GHz. From the separation between the lowest symmetric and antisymmetric inversion energy level of E = 760.77062(20) MHz a barrier to inversion, using a standard analytical model potential, of 1531 cm(-1) (18.31 kJ/mol) was deduced. The (14)N quadrupole coupling constants were found to be χ(aa) = 2.67576(37) MHz, χ(bb) - χ(cc) = 4.34144(65) MHz, and |χ(bc)| = 2.9199(92) MHz. The internal rotation of two equivalent methyl groups gave rise to very narrow multiplets; the torsional barrier was determined to be 1051.74(57) cm(-1). The experimental results were supplemented by quantum chemical calculations.