This work provides an integration of the financial and economic aspects of the interest rate. It depicts how the interest rate operates in the macro-economy to set the supply and the allocation of capital and how it functions on the micro-economic level to optimize capital decision-making. It describes the function of the financial markets in setting the supply and cost-of-funds to various users, of estimating a reasonable cost of capital for operational purposes of business firms and public sector authorities, and the application of discounting and present-value methods to financial and investment decisions and strategies. This work is unique in presenting seamless coverage of the function of the interest rate across varying markets. It will be of interest to scholars and students in economics and finance.