This paper provides a conceptual review of how the impact of taxes on the incentive to invest in the corporate sector can be measured. The focus is on measures derived from economic theory. Two measures are derived - effective marginal and average tax rates - which reflect different forms of investment decisions. A number of extensions to the basic model are examined, including the role of personal taxes, the source of finance and risk. These measures are compared to empirical measures based on observed tax revenues or tax liabilities.