Theoretical and experimental work completed during the past decade indicates that interstellar propulsion systems based on the annihilation of antiprotons in a propellant will be feasible during the next several decades. The major technical dif culty appears to be the ef cient production of antimatter. Estimates based on the use of currently available technologies indicate that the equipment required to produce and store antimatter would be inef cient and expensive. Hence, potentially more ef cient processes should be examined. Currently, antiprotons are produced by colliding high-energy protons with stationary heavy-element targets, such as tungsten. Other approaches, for which experimental data are available, indicate that there are production methods that are more ef cient. Several of these have been examined, and some show a signi cant improvement in ef ciency, but they may be dif cult to implement. Two methods examined will be discussed along with some technical advantages and disadvantages. In the rst method, antiprotons and pions are collected from the collision of high-energy protons and a heavy-element target. Many more pions are produced than antiprotons. The pions are then directed toward the same target or a different target. The collision of pions and heavy nuclei have a higher probability for the production of antiprotons and would signi cantly increase the number of antiprotons produced. In the second method, a recirculating electron/positron collider would produce multiple collisions near a resonance for producing antiprotons by using beam wigglers as in free-electron lasers. This technique would allow a signi cant increase in the number of interactions that would occur and would proportionally increase the antiproton production. Nomenclature c = speed of light Ç E 0 = energy storage rate e ¡ = electron e + = positron I = beam current J / w = charm-anticharm resonance particle K + , K ¡ = kaons L 0 = luminosity l = beam interaction length Ç M = mass injection rate m p = proton rest mass n = neutron P 0 = power p = proton p = antiproton q e = electron charge S 0 , s = center of mass energy a = constant parameter c = photon e , g = ef ciencies h = scattering angle l + , l ¡ = muons m e ,m e = electron neutrinos m l ,m l = muon neutrinos p + , p , p 0 = pions q 0 = rho resonance particle r , r w = cross sections