Phosphine gas has become the world’s primary fumigant for disinfestation of stored grain. Development and increase of strong resistance to phosphine is a threat to the continued use of phosphine and there are limited alternatives currently available. Understanding how phosphine resistance arises is critical in managing resistance problems and extending the useful life of this fumigant. The most recent advances have been in our understanding of the two genes that contribute to phosphine resistance in all pest species that have been examined in countries around the world. Resistance gene identification has led to the development of a monitoring tool based on the DNA sequence. Large surveys to test for resistance genes coupled with population genetics has provided us with information on how resistance arises and how it spreads.