Background: This study focuses on construct representation and inter-reader agreement and reliability in ePortfolio assessment of 1,315 writing portfolios. These portfolios were submitted by undergraduates enrolled in required writing seminars at the University of Pennsylvania (Penn) in the fall of 2014.  Penn is an Ivy League university with a diverse student population, half of whom identify as students of color. Over half of Penn’s students are women, 12% are international, and 12% are first-generation college students. The students’ portfolios are scored by the instructor and an outside reader drawn from a writing-in-the-disciplines faculty who represent 24 disciplines. The portfolios are the product of a shared curriculum that uses formative assessment and a program-wide multiple-trait rubric. The study contributes to scholarship on the inter-reader reliability and validity of multiple-trait portfolio assessments as well as to recent discussions about reconceptualizing evidence in ePortfolio assessment.  Research Questions: Four questions guided our study: What levels of interrater agreement and reliability can be achieved when assessing complex writing performances that a) contain several different documents to be assessed; b) use a construct-based, multi-trait rubric; c) are designed for formative assessment rather than testing; and d) are rated by a multidisciplinary writing faculty?   What can be learned from assessing agreement and reliability of individual traits? How might these measurements contribute to curriculum design, teacher development, and student learning? How might these findings contribute to research on fairness, reliability, and validity; rubrics; and multidisciplinary writing assessment? Literature Review: There is a long history of empirical work exploring the reliability of scoring highly controlled timed writings, particularly by test measurement specialists. However, until quite recently, there have been few instances of applying empirical assessment techniques to writing portfolios.  Developed by writing theorists, writing portfolios contain multiple documents and genres and are produced and assessed under conditions significantly different from those of timed essay measurement. Interrater reliability can be affected by the different approaches to reading texts depending on the background, training, and goals of the rater. While a few writing theorists question the use of rubrics, most quantitatively based scholarship points to their effectiveness for portfolio assessment and calls into question the meaningfulness of single score holistic grading, whether impressionistic or rubric-based. Increasing attention is being paid to multi-trait rubrics, including, in the field of writing portfolio assessment, the use of robust writing constructs based on psychometrics alongside the more conventional cognitive traits assessed in writing studies, and rubrics that can identify areas of opportunity as well as unfairness in relation to the background of the student or the assessor. Scholars in the emergent field of empirical portfolio assessment in writing advocate the use of reliability as a means to identify fairness and validity and to create great opportunities for portfolios to advance student learning and professional development of faculty.  They also note that while the writing assessment community has paid attention to the work of test measurement practitioners, the reverse has not been the case, and that conversations and collaborations between the two communities are long overdue. Methodology: We used two methods of calculating interrater agreement: absolute and adjacent percentages, and Cohen’s Unweighted Kappa, which calculates the extent to which interrater agreement is an effect of chance or expected outcome. For interrater reliability, we used the Pearson product-moment correlation coefficient. We used SPSS to produce all of the calculations in this study.  Results: Interrater agreement and reliability rates of portfolio scores landed in the medium range of statistical significance.  Combined absolute and adjacent percentages of interrater reliability were above the 90% range recommended; however, absolute agreement was below the 70% ideal.  Furthermore, Cohen’s Unweighted Kappa rates were statistically significant but very low, which may be due to “kappa paradox.” Discussion: The study suggests that a formative, rubric-based approach to ePortfolio assessment that uses disciplinarily diverse raters can achieve medium-level rates of interrater agreement and reliability. It raises the question of the extent to which absolute agreement is a desirable or even relevant goal for authentic feedback processes of a complex set of documents, and in which the aim is to advance student learning. At the same time, our findings point to how agreement and reliability measures can significantly contribute to our assessment process, teacher training, and curriculum. Finally, the study highlights potential concerns about construct validity and rater training.  Conclusion: This study contributes to the emergent field of empirical writing portfolio assessment that calls into question the prevailing standard of reliability built upon timed essay measurement rather than the measurement, conditions, and objectives of complex writing performances.  It also contributes to recent research on multi-trait and discipline-based portfolio assessment.  We point to several directions for further research:  conducting “talk aloud” and recorded sessions with raters to obtain qualitative data on areas of disagreement; expanding the number of constructs assessed; increasing the range and granularity of the numeric scoring scale; and investigating traits that are receiving low interrater reliability scores. We also ask whether absolute agreement might be more useful for writing portfolio assessment than reliability and point to the potential “kappa paradox,” borrowed from the field of medicine, which examines interrater reliability in assessment of rare cases. Kappa paradox might be useful in assessing types of portfolios that are less frequently encountered by faculty readers. These, combined with the identification of jagged profiles and student demographics, hold considerable potential for rethinking how to work with and assess students from a range of backgrounds, preparation, and abilities.  Finally, our findings contribute to a growing effort to understand the role of rater background, particularly disciplinarity, in shaping writing assessment. The goals of our assessment process are to ensure that we are measuring what we intend to measure, specifically those things that students have an equal chance at achieving and that advance student learning.  Our findings suggest that interrater agreement and reliability measures, if thoughtfully approached, will contribute significantly to each of these goals.