While it is generally accepted today that international humanitarian law (IHL) is binding on organized armed groups, it is less clear why that is so and how the binding force of IHL on organized armed groups is to be construed. A number of explanations for that binding force have been offered. The present contribution critically examines five such explanations, namely that organized armed groups are bound via the state on whose territory they operate; that organized armed groups are bound because their members are bound by IHL as individuals; that norms of IHL are binding on organized armed groups by virtue of the fact that they exercise de facto governmental functions; that customary IHL is applicable to organized armed groups because of the (limited) international legal personality that they possess; and that organized armed groups are bound by IHL because they have consented thereto.