General ecological theories have paid scant attention to the information preserved in the fossil record. However, in order for an ecological theory to be truly general, it must hold in any ecosystem at any point in time. Here, we make the case that all modern ecological theories should be tested in geological time.We explore some of the limitations of the fossil record when examined in light of modern ecology. While there are fundamental differences between the way modern ecosystems and fossil ones are studied, we demonstrate that comparisons between the two are not impossible. We present three major research areas where fossil information has been successfully used to inform modern ecological thought; namely community ecology, biogeography and extinction studies. These examples also serve to highlight ecological issues that could not have been conceived purely on the basis of modern data. We advocate a much stronger interaction between modern ecologists and paleontologists in addressing present and future ecological questions.