Although there has been growing interest in suggestions that geomagnetic reversals may be initiated by external sources, we show that the present evidence favors a mechanism of internal origin for at least the vast majority of reversals. Several lines of evidence indicate that the dipole and quadrupole dynamo families are decoupled most of the time but are coupled across polarity transitions. This suggests a reversal model in which reversals are initiated when the quadrupole family couples critically with the dipole family. ¿ American Geophysical Union 1988