The possible structures involved in the ring penetration and windowing mechanisms for the formation of endohedral fullerenes are reported. Results for X@C-60 relaxed transition structures and energy barriers are reported, where X = H, He, N, NO, P, and As. The applicability of a mechanism to formation of a particular endohedral species is readily judged on the basis of these barriers, with only hydrogen, helium, and nitrogen as likely participants in direct insertion. Windowing was shown to be feasible only for hydrogen or nitrogen atoms under the adiabatic conditions of the calculations.