This paper describes the construction of a financial stress index (FSI). Our index incorporates the level, volatility and comovement of a variety of financial series, rather than a single dimension of the data. To determine which time periods are ones of notable financial stress and thus the relevant ones for determining the role of the level, volatility and comovement of our financial series, we use actions taken by policymakers. In addition to describing the construction of our FSI, we discuss issues relevant to the general construction of stress indexes such as how an FSI differs from a financial conditions index, the challenges of combining different financial series into a single measure and the role historical experience plays in index construction.