This thesis presents an alternative approach to capital budgeting, named Fuzzy Modified Methods of Capital Budgeting, for evaluating investment projects under uncertainties. The development of the proposed approach is divided into two phases: in the first stage, a general deterministic model that explicitly provides for the use of the opportunity costs associated with the interim cash flows of a project is established. The implicit assumptions of the modified internal rate of return and modified net present value methods are included in the index of profitability and in the total payback period. The resulting indicators are the modified index of profitability and the modified total payback period. This unified approach has the property to match the decisions of acceptance / rejection of investment projects with same horizons of life and same scales with the decisions of the modified net present value method and therefore maximize shareholder wealth. In the second phase, triangular fuzzy numbers are used to represent the uncertainties of the project variables: cash flows and reinvestment, financing and risk-adjusted discount rates. The resulting indicators are the fuzzy modified net present value, the fuzzy modified internal rate of return, the fuzzy modified index of profitability and the fuzzy modified total payback period. The application of opportunity costs and fuzzy criteria for determining the variables allows obtaining more realists and consistent results with the market conditions. Due to the complexity of the calculations involved, new MS-Excel financial functions are developed by using Visual Basic for Applications: three functions for evaluating projects under conditions of certainty (MVPL, MIL and MTRI) and four functions for evaluating projects under uncertainties (MVPLfuzzy, MTIRfuzzy, MILfuzzy and MTRIfuzzy). The main contribution of this thesis is to develop a unifying approach to capital budgeting under uncertainty that emphasizes the strengths of the methods of modified net present value and modified internal rate of return, while bypassing the individual conflicts and drawbacks of the conventional methods. Results show that the proposed methods are more advantageous and simpler to use than other methods of investment appraisal under uncertainties.