This chapter introduces how a language program can utilize portfolio assessment to measure multilingual students’ linguistic and cultural development holistically. It showcases students’ learning outcomes with examples, and discusses the benefits, applications, and challenges of portfolio assessment. This chapter argues that portfolio assessment can transform students from passive learners or test receivers in the classroom to become active participants, thinkers, and assessors in charge of their own learning. More importantly, portfolio assessment facilitates multilingual students’ linguistic and cultural competence, advances their critical thinking and metacognitive skills, promotes student agency, and benefits their long-term growth. Likewise, in language classrooms, the portfolio process can (re)construct multilingual students’ self-confidence and empower them when they utilize their existing linguistic and cultural repertoires to acquire and practice new knowledge. This chapter suggests that language practitioners and their classroom activities should focus on developing multilingual students’ translanguaging and cross-cultural competence to enable them to survive and thrive in an increasingly multilingual and multicultural world.