This new edition has been thoroughly revised, expanded and updated. In particular, it contains an entirely new section on the Selection of Cost-Benefit Criteria which illuminates the basic rationale of the economic calculus. In addition, the second half of the section on Investment Criteria has been radically altered to produce a more crisp and lucid treatment of the essentials. It contains a general critique of the DPV method with regard to the more recent contributions to this controversial literature. Finally this new section tackles the inter-generation problem involved in long-term projects, a problem that has so far been evaded in the literature on investment criteria.