Modern agricultural commodity markets are simultaneously governed by a physical and a financial market element. Whether financial “index trading” activity influences price levels on the futures markets has been investigated by empirical studies using Granger Causality Analysis. A critical review of these studies reveals inconclusive results. Based on sensitivities of the method, reasons for limited interpretability of results may be omitted determinants of financial trading activity, failure to consider the informational efficiency of markets, time-varying and feedback effects of boundedly rational heterogeneous trading strategies and limitations in specifying adequate theoretical variables from existing data.