When computer simulations are popular in helping students understand chemistry in today's classrooms, it is important to realize how instructional use of computer simulations affects students' understanding of science. This dissertation centers around the impact of the use of computer simulations on college students' learning. Chapter 1 generally addresses the background and the significance of the research topics. Chapter 2 reviews the literature from research that studied the factors that affect the use of computer simulations in helping students learn science. Learners were found to understand science theories better with descriptions and explanations presented in both verbal and visual formats than in verbal format alone. An individual's prior knowledge and learning strategies have also been found to have an impact on her/his response to computer simulations and therefore affect the potential value of computer simulations. Chapter 3 reveals the impact of the use of computer simulations on students' understanding of electrochemistry principles. The results confirm findings in earlier studies that college students seemed to be able to build mental models of chemical reactions from formula and equations with or without the help of computer simulations. The study in Chapter 3 indicates that it is likely that the design of the learning activities rather than the use of technology actually had an impact on students learning. Chapter 4 provides insights into how the use of simulations affected the communication between group members and how individuals with different levels of prior knowledge responded to computer programs and interacted with peers. Although prior knowledge was not found to interact with the use of computer simulations in affecting students' understanding, the findings in Chapter 4 show that prior knowledge seemed to affect the ways that students solved problems and the ways they interacted with the computer simulations.;Taken together, these three studies in this dissertation suggest continuing research needs to be done in identifying and resolving issues when individual differences are considered. In addition, it is important that the design of learning activities be given a higher level of priority than the use of instructional technology when employing computer simulations in the classrooms.