The philosophic and practical aspects of intergenerational planning for a 50 – 100-year time frame are reviewed, with recognition of its speculative quality. Society's near term choice of future physical pathways based on comparative quantitative benefit/cost/risk analyses of alternatives is usually modified by the intervention of a variety of time-dependent, nontechnical value systems. Further, the continuous competition among society's disparate technical systems, capital investment choices, and planning objectives all contribute to the uncertainty of the intergenerational outcome of any plan. Nevertheless, the quantitative planning process provides an essential base. Benefit/cost/risk projections are discussed for both the case with a historical database and the case without such a historical base. The end-objectives and continuous nature of such benefit/cost/risk analyses are described.