The reaction front hypothesis in solid-state anaerobic digestion could have major implications for the seeding of both landfills and packed-bed digesters. Firstly, the formation of reaction fronts would require seed bodies of a size that might be rare in wastes. Secondly, as seed bodies of viable size are likely to be effectively immobile, their initial distribution pattern would determine the process kinetics. In the light of these possibilities, the conclusions drawn from many laboratory and site-scale studies require reconsideration. Moreover, even the best current seeding practice might be sub-optimal. Packed-bed digesters are usually well seeded but optimized seeding might allow the use of cheaper plant. Landfills, however, are very rarely seeded. Optimized seeding is expected to initiate a predictable and relatively quick process at little or no cost, with an acceptably low seeding rate, a shorter lag phase and a constant-rate output of biogas throughout the period of stabilization.