A promising source of insights into heritage language learning comes from the broader study of the role of input in language acquisition. We concentrate here on the possibility that qualitative differences in the proficiency of heritage and monolingual language learners can be traced to a qualitative difference in the input available to each group. By examining a series of representative phenomena (case, differential object marking, and disjunction), we illustrate how shortfalls in experience can shed light on the many instances of partial acquisition and of attrition that are reported in the literature on heritage language learning. (Contains 3 figures, 5 tables and 7 notes.)