This chapter brings together the findings from the three studies, which confirmed that Differential Object Marking (DOM) is a vulnerable grammatical area not only in Spanish, but also in Hindi and Romanian as heritage languages, subject to erosion under pressure from, English in this case. This chapter goes deeper into these overall trends, by comparing the three heritage speaker groups, on the one hand, and the three first-generation immigrant groups, on the other, on several background variables related to patterns of language use. A follow-up replication study with Spanish heritage speakers and immigrants from other countries in Latin America is reported, which confirm the attrition effects in the two generations of Mexican immigrants. It is claimed that his finding is strong evidence that while DOM omission may have started as a developmental outcome of heritage language acquisition, it may be on its way to becoming a stable dialectal feature of Spanish in the United States, suggesting language change with respect to DOM in Spanish. The roles of language internal and language external factors are discussed.