The low frequency dielectric relaxation (1 to 10⁵ Hz) of dipolar systems has been studied in the supercooled liquid state. Studies were made on fluorenone/o-terphenyl, fluorenone/o-terphenyl/triphenylbenzene, fluorenone/o-terphenyl/decalin, and di-n-butyl phthalate/o-terphenyl systems, where the latter system was studied from dilute solution to pure di-n-butyl phthalate. The results show that the dipolar molecules move co-operatively with their environment in the supercooled liquid state, and the derived dipole moment autocorrelation function has a natural non-exponential dependence upon time. It was shown that the fluorenone reorientation rate in o-terphenyl could be made faster by addition of small amounts of decalin, and could be made slower by an addition of small amounts of triphenylbenzene. The concentration dependence of the shape of the relaxation of di-n-butyl phthalate is quite different from that of fluorenone in o-terphenyl, and is analyzed to yield the effective distribution of local concentration surrounding a dipolar molecule. The relationship between dielectric relaxation and other techniques which detect molecular motion is briefly discussed, and the dipole correlation function for dilute supercooled o-terphenyl solutions is used to predict [〈3 cos²θ(t)〉—1]/2 for these systems.