The recognition that predators play important roles in ecosystems has prompted research to resolve how combinations of predator species influence ecosystem functions.  Interactions among predator species and their prey can lead to a host of linear and nonlinear effects.  Understanding the conditions causing these effects is critical for assigning predator species to functional groups in ways that lead to predictive theory of predator diversity effects on trophic interactions. To this end, I provide a synthesis of experiments examining multiple predator species effects on mortality of single shared prey.  I link natural history insights on predator species habitat and hunting behavior with linear and nonlinear multiple predator effects to derive a new concept of predator diversity effects on trophic interactions.  This concept holds that the nature of predator diversity effects is contingent upon predator species hunting mode plus predator and prey species habitat domain (defined as the spatial extent to which a microhabitat is used by a species).   This concept allows the classification of multiple predator effects into four broad functional categories (substitutable, nonlinear due to predator species interference, nonlinear due to intraguild predation, and nonlinear due to predator species synergism).  Experimental evidence so far provides ample and comparatively equal support for substitutable, interference and intraguild effects and equivocal support for nonlinear synergisms.