This chapter provides a case study of the process of endangered language acquisition, which has not been well studied from the viewpoint of applied linguistics. It describes the context of teaching Chinese adult learners in Taiwan an endangered indigenous language, the teachers’ pedagogical approaches, the phonological and syntactic acquisition processes the learners were undergoing, and applications to other language documentation and revitalization programs. Both qualitative and quantitative methods were used to address the research questions. This study demonstrates cogently that language is a complex adaptive system. In phonological acquisition, the trill was the most difficult phoneme to learn. Systematic variations for the variables (ŋ) and (s) were found to be constrained by both markedness and interference. Furthermore, learners also tended to interpret Yami orthography based on their knowledge of English. In word order acquisition, learners performed much better than expected, partially because the present tense, coded by the SV word order, is the norm in Yami conversations. However, students still inaccurately associated word order with sentence type rather than with tense distinction. The Yami case provides an integrated model for endangered language documentation, revitalization and pedagogical research, which would be of interest to people working with other languages and the language documentation field in general.