We investigate the analytical and empirical linkages between cash flow, uncertainty and firms' capital investment behavior. Our empirical approach constructs measures of own-and market-specific uncertainty from firms' daily stock returns and S&P 500 index returns along with a CAPM-based risk measure. Our results indicate that even in the presence of important firm-specific variables, uncertainty is an important determinant of firms' investment behavior. Depending on the measure of uncertainty used, investment may be stimulated or curtailed by the effects of uncertainty on its own or through its interactions on cash flow.Keywords: capital investment, cash flow, uncertainty, CAPM JEL: E22, D81, C23 * We are grateful for comments received from seminar participants at Koç University, the University of Strathclyde and the 13th International Conference on Panel Data, University of Cambridge, Fabio Schiantarelli and the constructive suggestions of an anonymous reviewer.