Graduate teaching assistants (GTAs) are an integral part of the instructional personnel in undergraduate general chemistry courses. Professional development programs for GTAs are well-established across STEM disciplines ranging from two-day to week-long sessions, covering a wide variety of topics, ranging from pedagogy and safety standards to duties, expectations, and resources as a university employee. Chemistry Education research (CER) is an evidence-based approach to chemistry teaching and learning. Assessment of weekly laboratory reports is one of the primary responsibilities of Chemistry GTAs. Professional development (PD) of GTAs specifically for assessment of student work is a largely unexplored area in CER. My doctoral research in CER aims to address this gap and explores professional development modules designed to help GTAs become reliable and consistent graders. This dissertation describes the design and implementation of specialized grading activities that enhance GTAs' understanding of grading criteria, chemistry misconceptions, and common technical errors in undergraduate students' writing. Program evaluation is a rare feature in such initiatives. This work is the first-known report of using growth models as a novel statistical approach for evaluation of GTAs or GTA training programs. I examine factors influencing chemistry GTAs' professional development as reliable graders. This longitudinal research was informed by extensive individual and group interactions with GTAs, needs assessments, technological, pedagogical, and online learning management system (LMS) tools to support GTAs in developing reliable and consistent grading skills. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]