The focus of my action research was the use of an electronic student response system during instruction in two second-grade classrooms. Each classroom was equipped with eInstruction's Classroom Performance System (CPS). Each teacher used CPS for five regular instructional lessons and kept a log pertaining to her use of CPS. I observed two lessons prior to the intervention period and four lessons during the intervention period. I hoped to determine the effectiveness of using CPS for assessing students' learning and for providing effective feedback to students throughout instruction. The results of my research indicate that use of CPS improves the teacher's ability to assess student learning and to provide feedback to the students throughout instruction. It was also noted that use of CPS increased student involvement and on-task behaviors and enabled all students to respond to the teachers' questions during instruction.