This study is primarily aimed to investigate beliefs and attitudes of Indonesian EFL preservice teachers toward the use of mobile devices such as smartphones or tablets for learning and instructional practice in Indonesia. Furthermore, this phenomenological study attempted to reveal the factors affecting these two constructs from 20 Indonesian EFL preservice teachers through semi-structured interviews. The findings revealed participants' beliefs that mobile devices could positively contribute to the development of language performance as learning tools and the development of language instruction as instructional tools. Regarding attitudes of EFL preservice teachers toward the use of mobile devices for learning and teaching, this study revealed mixed responses. As learning tools, all participants expressed their positive attitudes toward this technology and intention to use this technology more intensively. As for teaching tools, the majority of the participants expressed their negative attitudes which were reflected in their disappointment through their experience during their student teaching program and their lack of interest to adopt this technology for their future classes. However, they acknowledged that this technology was helpful to facilitate online learning and agreed that this technology would become more popular in the future. Analysis of the data also revealed factors affecting beliefs and attitudes of preservice teachers toward the use of mobile devices as learning and instructional tools comprising perceived ease of use and perceived usefulness and other influencing elements of self-efficacy: enactive mastery, vicarious experience, and physiological arousal. Finally, this study presents implications and recommendations which can be a reference to optimize the integrated mobile technology for class instruction and identifies which aspects in the implementation of mobile technology that require further investigation. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]