This research investigates whether the flipped classroom can lead students to increased gains on learning outcomes in 2 ESL/EFL contexts in Macau, China, and the US. A pretest posttest quasi-experimental mixed-methods design (N = 64) was used to determine any differences in student achievement that might be associated with the flipped approach (FA). The effectiveness of the FA on students' achievement with grammar-student learning outcomes was evaluated with a pretest and posttest grammar test, along with students' perceptions of their increased comfort and confidence using English grammar through a grammar survey. These data were triangulated with student focus groups and means of completed grammar assignments. The findings suggest that although both the control and experimental groups showed increased comfort in the self-report data, gains on actual achievement were significant only for the experimental groups. The researchers of this study make recommendations for a flipped curriculum and materials design for ESL/EFL teachers in any context globally.