Over the past three decades, terrestrial magnetospheric physics has had a unifying and hotly debated focus: the magnetospheric substorm. A magnetospheric substorm is a three-phase phenomenon [McPherron, 1979] in which energy is first extracted from the solar wind flow, transported, and stored within the Earth’s magnetospheric magnetic fields (growth phase). The stored magnetic energy is then converted and released explosively within the magnetosphere and auroral ionosphere (expansion phase). Then the magnetosphere and auroral ionosphere relax, entering a quiescent state (recovery phase).Understanding of this global process has unified the magnetospheric and auroral communities by providing the “big picture” of magnetospheric dynamics as a backdrop against which unrelated areas of more focused research may be put in context. The topic debated in the accompanying articles centers on one aspect of magnetospheric substorms, namely what triggers the expansion phase. The companion papers present two perspectives on substorm triggers: one advocates a trigger that is driven by an external change in the interplanetary magnetic field (IMF), while the other argues that external triggers are just one of many mechanisms that lead to the expansion phase, including internal instabilities.