This document summarizes research on the acquisition of grammatical morphemes by adult second language learners. It finds that while the order of acquisition is not identical to that of children acquiring their first language, there are clear similarities. Specifically, it is found that adult second language learners display a "natural order" of acquisition for certain grammatical structures, like copula and auxiliary verbs, similar to that seen in children. However, this natural order can be disrupted when learners focus more on conscious learning and monitoring of language forms. Tests were developed to measure whether learners were relying more on acquired or learned systems. While the existence of a natural order is debated, most research agrees it reflects the unconscious language acquisition process in adults