The portfolio is considered a useful tool both for instruction and assessment. Properly designed and implemented, it provides authentic language material for assessment, increases learners' involvement in learning process and promotes self-reflection. This article mainly reviews the empirical research on portfolios in ESL/EFL context and offers suggestions for future research. The article starts by providing a brief introduction to portfolio and the framework for systematically designing and implementing portfolio assessment in the classroom. Then it reviews the empirical studies of portfolios in ESL/EFL context from three perspectives, i.e. portfolio assessment on writing, portfolios as a means to promote autonomy and e-portfolios. The article concludes by emphasizing the benefits of portfolios in language learning, indicating challenges in carrying out portfolio assessment, and providing suggestions for future research.