||The purpose of this study was to investigate mainstreamed ESL students' challenges in higher education and the process they went through for adjustment to mainstream classes. Especially, I addressed five Korean ESL students' experiences at colleges / universities in the U.S. I conducted a focus group interview along with a self-assessment questionnaire as data gathering methods. From data analysis, I noticed that all the participants in this study experienced transformative changes while trying to adapt to the new learning environment in mainstream classes. Challenges such as different classroom norms, different learning style, cultural / individual differences, and English deficiency in academic area in mainstream classes acted as crises (disorienting dilemma) and directed these students to question their existing assumptions (critical reflection). Consequently they gained new transformed perspectives, which they integrated in their lives to deal with challenges in the adjustment process. The findings suggested that many factors were interwoven to affect these students' adjustment process besides their language related problems. For some solutions to help ESL students cope with challenges in mainstream classes, I presented four ESL specialists' suggestions of possible strategies in classroom practice and institutional policy.