In this qualitative study, we examine how students' participation in the process of constructing portfolios may affect their motivation for and engagement in literacy learning. Research that is based on the “wisdom of practice” continues to emphasize the value of portfolios and portfolio-based assessment. Few studies, however, articulate an explicit theoretical frame for viewing portfolios and considering their effectiveness. This study grounds portfolio practices in 2 bodies of literature that lend theoretical support to their importance, and then extends outward to include literature that addresses the social construction of meaning and the situated nature of learning. Using vignettes of students from an urban high school, we lay out our initial understandings of portfolios and our a priori assumptions about the concepts of motivation and engagement; we articulate our emerging theory around process portfolios, and trace our processes of reading and interpreting the data; we explain how our theory has evolved and lead us to a new set of literature that broadens the focus of motivation and engagement to include not only the individual student but also other sociocultural factors; and, finally, we discuss the implications of this work for practitioners, researchers, and policy makers.