Writing performance is essential for academic success at all levels. To help elementary school students become better and motivated writers, educators need to help them have functional writing self-efficacy beliefs that slightly exceed what a learner can actually accomplish. A suggested way to achieve this is having students create process portfolios to: a) document the writing process and their progress monitoring, b) have access to peers' work and c) receive feedback. This multiple case study explored the development of students' writing performance and writing self-efficacy beliefs through a one-academic-year implementation of process portfolios in three fourth grade elementary school classes (N=63 students) in Cyprus. The students of two fourth grade classes (n2=23, n3=20) created paper-based process portfolios, while in the researcher's class (n1=20) students created digital portfolios. The study explored how process portfolio affordances, such as a process approach in writing, progress monitoring (goal setting, reflection, self-evaluation), access to peers' work and feedback related to students' writing performance and self-efficacy and how this relationship changed over time. It relied on a mixed method (quantitative and qualitative) research methodology comprised of pre- mid-and post- portfolio implementation students' writing performance and self-efficacy tests, teachers' and students' interviews on their perceptions of portfolios and a portfolio artifact analysis. While there is much to understand regarding the impact of portfolios on students' writing performance and writing self-efficacy the results of this study produced several important findings and practical implications related to portfolio use and the added pedagogical benefits of portfolio affordances. The key findings revealed that students' writing performance increased over time. Learning gains were also found in students' ability to provide corrective feedback and constructive comments to their peers' work, to set goals, and to provide accurate self-evaluations of their work. Students' writing self-efficacy increased over time and became more accurate as it reflected students' actual performance by the end of portfolio implementation. Implications for practice and guidelines based on a bottom-up approach that could facilitate a large-scale implementation of digital and paper-based portfolios in Cyprus in the future are offered