This book reports on a study aimed at exploring the relationship between Iranian junior high and high school students' language proficiency level and their beliefs about second language acquisition. Moreover, this study identified and compared the difference between the SLA beliefs of Iranian students with different gender, grade, language experience, and family language backgrounds. This study contributes to the growing understanding of the role of beliefs in language learning. Further, the results of this study can be of importance in explaining the relationship between learning beliefs and language proficiency level, as well as in providing English teachers in Iran with a better understanding of Iranian students' about English learning.