The current study has a multi-method research design and investigated portfolio assessment comprehensively in three parts. First, teachers’ portfolio practices and views were investigated by means of semi-structured individual interviews in a qualitative design. Secondly, portfolio contents were examined with a developed content checklist and rubric under the methodology of document analysis. In these parts, 24 teachers participated and 19 child portfolios were examined in a Reggio Emilia-inspired preschool and university preschool in Turkey and the U.S. In the final part, teachers’ portfolio assessment practices and related predictors were investigated using a quantitative methodology. Data were collected from 605 early childhood teachers in Turkey, scales were developed for data collection, and descriptive statistics, MANOVA, and Hierarchical Multiple Regression were utilized in data analysis. Qualitative findings revealed that portfolio practices meet on similar points in the same country rather than in the same preschool type. Teachers expressed several benefits of portfolio assessment for each stakeholder including the child, teacher, and parents. Document analysis supported interviews findings, and it was seen that portfolios serve the purpose of assessment when evaluated overall. However, those need to be enriched in terms of child reflection. Finally, through quantitative analysis, it was found that intention and self-efficacy beliefs are the significant predictors of teachers’ portfolio practices, and teachers’ portfolio practices do not change significantly with respect to teaching experience. Furthermore, teachers practicing portfolio assessment were also found to have higher scores in internal constructs (e.g. attitudes, beliefs) compared to others who are not using portfolios.