The current tax code is the product of an ongoing legislative process influenced both by shifts in the philosophy of taxation and by growth in understanding the economic implications of taxation. The result is an extraordinarily complex code that is frequently at cross-purposes with itself. This report highlights the major trends in the U.S. tax system since the beginning of the income tax, and especially over the last several decades, to illustrate how we arrived at the current tax system. Such an historical perspective on the tax system is crucial for understanding the motivations of features of the current code and evaluating proposals for simplification and reform.