In writing science summaries, student writers frequently borrow language fragments from source texts. While taking a text's ideas verbatim is commonly considered a failure in writers' expected use of their own words or even plagiarism, imitating "linguistic chunks" from skilled speakers is also an effective practice in language development. This study argues for the need to investigate textual borrowing (TB) from a developmental perspective. A total of 956 U.S. upper-elementary and middle school L1 students were administered a reading comprehension test, a written summary task, and a receptive academic language assessment. Multiple indices were generated to capture TB frequency, length, and one particular function of TB (i.e., definitions). Results revealed that 68% of summaries contained at least one incidence of TB. As expected, both TB frequency and length were negatively associated with reading comprehension and academic language skills. However, students in higher grades demonstrated significantly more and longer TB in comparison to their younger counterparts when summarizing the same passage. In addition, the frequency of borrowed word strings and the ratio of definitional borrowing were positively associated with summary writing quality, and the latter association was found to be stronger for students with higher academic language skills. Implications for pedagogy and future research are discussed. [For the corresponding grantee submission, see ED614239.]