Computers have taken an increasingly prominent role in education around the world in recent years in developed and developing countries alike. As developing country governments have turned their focus from increasing enrollment to improving the quality of education in their schools, many have made access to computers a key component to their strategies (Trucano, 2005). There is an emerging body of research now that shows that these strategies, which often come with a hefty price tag, have varied effects (Glewwe et al., forthcoming). This paper contributes to this literature by comparing the use of computer-assisted language learning software to traditional methods of English instruction in Costa Rica. The authors also compare two different software programs to one another. Because schools were randomly assigned to one of these two treatment groups or a control group, this research permits a rigorous comparison of the differential effects of two software programs, holding contextual factors constant. The following research questions are addressed: (1) What is the impact of each of the two English language learning software programs on test scores, as compared to a teacher alone; (2) what is the magnitude of the effect of each program compared to the other; and (3) do these effects vary by school-level baseline performance, students' baseline test scores or gender? The main finding of this research is that academic software can be an effective learning tool, but that this depends on the software. Previous research has already shown that technology can be effective in some cases and ineffective in others. One of this paper's contributions is to show that these heterogeneous effects are not simply the product of using technology in different contexts (although that is likely to be important as well). By randomly assigning two different software packages to students in similar schools, this research has shown that the type of technology used matters, holding other factors constant. Furthermore, technology's effectiveness also depends on student characteristics like baseline abilities and gender. Tables and figures are appended.