is to improve the overall return per unit of risk in a client's portfolio through active management of asset allocation deviations. A GTAA strategy alters the asset allocation in the portfolio both to reduce a portfolio's unintentional asset allocation risk and to add excess return through intentional asset allocation deviations. Because the strategy's goal is twofold, it is often constructed as two separable pieces: (1) a rebalancing or completion portfolio, of which the sole objective is to reduce asset allocation risk in the portfolio, and (2) a pure overlay portfolio that is designed to capture excess return through global tactical asset allocation deviations. Both the completion and pure overlay elements of a GTAA program can be customized to the specific needs of every portfolio. These include the portfolio's strategic asset allocation, its existing active and passive investment portfolios, and also client-specific objectives and investment constraints such as targeted active risk and constraints on leverage or position sizes. The degree of customization in GTAA often makes the strategy difficult to classify into a particular asset class, as the uses of it differ so widely across investors' portfolios. The purpose of this chapter is to set forth the key elements of GTAA, while at the same time explaining the nuances of GTAA that make the strategy so appealing to various clients. In the following pages, we walk through the modern GTAA strategy in detail, explain and show through examples how clients and investment managers should best implement GTAA, and demonstrate how powerfully a GTAA program improves a portfolio's risk/reward profile. After a brief history of GTAA, the chapter continues in four primary sections: the structure of a GTAA program, the empirical evidence supporting GTAA, practical implementation issues that face clients and managers, and finally, expectations for a GTAA program. HISTORY OF GTAA As long as investors have been buying and selling securities, they have also been timing the market. One of the early and great market timers was Charles Henry