This chapter looks at how the study of native language (L1) phonological attrition reveals that the system of phonological contrasts of the L1 may change as a consequence of new language (L2) acquisition in adulthood. It also reveals that changes at the system level may underpin the variable pronunciation patterns that have been observed in phonetic studies at the local level of individual segments or features. Such changes may include contrast blurring as well as boosting of the distinctiveness among system elements as a reaction to contact with a different phonological system. In addition to speech production, perception is also affected by the effects of phonological changes in L1 attrition, showing how pervasive the attrition phenomena may be for both speaking and listening abilities