This diploma thesis explores the portfolio, a tool for developing learner self-assessment, in English language classes. The theoretical part focuses on the introduction of necessary terms such as assessment and evaluation, self-assessment, portfolio in the context of both general education and English language teaching. Furthermore, the thesis proposes a critical insight on these terms. The practical part presents an action research with the aim to incorporate the European Language Portfolio into a lower-secondary English classroom and to describe the whole process in detail from a teacher's perspective