This mixed-methods study explored the effects of student-generated visualization on middle-schoolers' science concept learning. We compared students who visualized during study time with those who did not and found that visualization as a study strategy led to students' improved test performances (p=.02). However, middle schoolers' scores on a test of science concepts did not improve as a result of a computer-based visualization workshop (p=.03). In fact, workshop participants scored lower on the test than did those who did not receive the workshop. Qualitative analysis identified elements in the school setting that interfered with instructional effectiveness in the computer-based workshop. Findings indicated that across groups students were quite unskilled at visualization. We found that visualization is a difficult but powerful study strategy and recommend that science curriculum focus on visualization of concepts. (Contains 2 tables and 6 figures.)