Dreissena polymorpha and Corbicula fluminea are among the most aggressive freshwater invaders world wide, and often dominate water bodies they invade. They occur in similar habitats, however, their tolerance and preference for certain characteristics of freshwaters differ in important ways, and they can have different impacts on the environments they invade. We identify similarities and contrast differences between these species, and highlight important questions yet to be addressed, including: the ability to link short-term laboratory findings to large scale and long-term effects of invasion, the consequences of invasion by both species together rather than considering each in isolation, and identification of local versus system-wide effects when these non-native ecosystem engineers invade.