Lymphatic filariasis and onchocerciasis are subject to major intervention programs by the WHO. The Onchocerciasis Control Programme in West Africa was launched 30 years ago and has led to considerable insights into the control of this infection. The Global Alliance to Eliminate Lymphatic Filariasis is a relatively recent control program with ambitious targets concerning its efficacy and its schedule. These expectations, however, are based on certain assumptions about the density-dependent processes of limitation and facilitation which determine eradicability: the levels of transmission thresholds and breakpoints. Here, we review these processes operating in filarial infections and show their impact on the persistence of the parasite, as well as pointing out those issues where more information is required to develop sound predictions about the eradicability of these infections.