此方法的立论是将投资的未来现金流量，全部折现成投资始日的价值，称为该投资的净现金流量，或称为净现值（NPV, Net Present Value）。
假设投资的净现值为正数，代表该投资的结果可以增加企业的价值;反之，如果投资评估的净现值为负数，代表此投资会减少企业的价值，不应该接受。

当然净现值为正数仍不代表该接受此投资建议，或许存在其他净现值更高的投资机会。在多项投资案并陈时，应选择净现值最高的投资案。净现值是多数企业进行投资评估时的依据，因为它将投资期间的所有现金流量纳入考虑，并且以折现后的金额做计算。

In finance, the net present value (NPV) or net present worth (NPW) is a measurement of the profitability of an undertaking that is calculated by subtracting the present values (PV) of cash outflows (including initial cost) from the present values of cash inflows over a period of time. Incoming and outgoing cash flows can also be described as benefit and cost cash flows, respectively.