This study examined whether teacher portfolios can be validly and reliably assessed by investigating the effect of an instructional tool on increasing the level of reflective thinking in elementary preservice teachers' portfolios. It also examined whether reflective thinking in preservice teachers' electronic portfolios represented sufficient quality to make them useful in practice. The "Rubric for Evaluating Portfolio Reflective Thinking" instrument developed for this study demonstrated moderate levels of interrater reliability (r = 0.66) and sufficient content validity to be used to measure reflective thinking. Also, members of the treatment group scored significantly higher on five of the six portfolio domains and on the total portfolio reflective score than members of the control group. Overall percentage levels of reflection were substantially higher for the treatment group (47%) than for the control group (6.7%). Implications for practice and further research are provided.