Syntactic complexity has been recognized as an important construct in writing research, and for the past five decades, many syntactic complexity measures (SCMs) have been examined in numerous studies. This systematic review is the first study of its kind to synthesize 36 studies spanning from 1970 to 2019 by identifying and cataloging all SCMs examined during this period. An analysis was performed on how the use of SCMs varied by genre, grade level, students’ writing ability, and writing quality. Five online databases (Academic Search Premier, ERIC, PsycINFO, Psychology and Behavioral Sciences, and PsycARTICLES) were searched. A total of 48 SCMs were grouped into six categories: T-units and sentences, clauses, phrases, words, combined measures, and other measures. Most studies examined three common SCMs: mean T-unit length, mean number of words per clause, and mean number of clauses per T-unit. The argumentative genre had the highest values for T-unit length and mean number of clauses per T-unit, which could indicate greater sentence complexity. Higher-grade-level students generally construct sentences that are syntactically complex, but comparison between studies was difficult because different studies investigated different SCMs. Although students with higher writing abilities generally construct sentences that are syntactically more complex compared to students with lower writing abilities, the findings are not conclusive, as only a few studies examined this relationship. A similarly inconclusive relationship was found between syntactic complexity and writing quality because only a few studies examined this relationship. More research is needed to examine the relationship between SCMs, writing quality, and genre.