of a portfolio's return are identified and measured. Attribution is a critical component of the quality control process within investment management and must be closely aligned with risk measurement. Optimal portfolio construction requires that exposures are created with risk proportional to the available opportunities to add value. Return attribution looks back and attempts to identify where and to what extent the exposures were successful. In order for this feedback process to be useful returns should be attributed as closely as possible to factors that fit into the portfolio manager's way of organizing and sizing risk exposures. Managers may rely on return attribution reports developed in-house or from commercially available systems. As for commercially available software, each system typically employs its own particular brand of attribution. Differences across systems can vary in certain ways, from the algorithms applied to the terminology used to describe the sources of return. The differences in algorithms and terminology can lead to confusion and make it difficult for managers to understand their portfolio's sources of return. Unfortunately, in many cases the return attribution system is a completely separate system from that used in risk measurement. When this is the case it may be difficult for the organization to make effective use of the information provided by the return attribution system. Suppose, for example, that a portfolio manager wants to invest in high-quality companies that have both growth potential and reasonable valuations. Suppose further that the manager has proprietary approaches to ranking companies along these dimensions. It would clearly be desirable to be able to measure to what extent the portfolio has exposure to these factors, and to monitor how much risk these exposures create and how much return these exposures have provided historically. Return attribution should answer this last question, and in order to do so, like a good risk system, it should be customizable to the process of the portfolio manager. This chapter presents a comprehensive review of some of the most commonly used methods for performing return attribution. Our focus is on equity portfolios although the results we present generalize to other asset classes. We explain the various methods that are employed by commercially available systems within a framework that uses common terminology and notation. The purpose of this chapter is threefold: to increase the transparency of return attribution computations, to