Teacher assessment literacy is highly regarded as one of the most influential factors in improving student learning (Black & Wiliam, 1999; Hattie, 2008). Researchers and educators argue that an assessment literacy program aimed at supporting teachers to enhance their assessment knowledge and skills should begin with measuring their current level of performance and identifying their training and support needs. There have been numerous attempts to develop and use tests, rating scales and other forms of assessments to evaluate teachers' assessment competence, but the real challenge lies in how to link the assessment results to professional development. Drawing on the work of Griffin (2007) which made a strong connection between Glaser's (1963/1981) and Rasch's (1960/1980) latent trait theory and Vygotsky's (1978/1986) zone of proximal development, and incorporating the works of Andrich (1978) and Masters (1982) in extending the probabilistic model of competence to polytomously scored items, this paper illustrates how the results of teachers' self-evaluation using a standards-based assessment for learning competency framework can be linked to professional development with an emphasis on training design, delivery, teacher learning, resource allocation and policy articulation. The paper goes on to illustrate how school systems can design and implement a professional development training framework to help language teachers gain a deeper understanding of sound assessment principles and practices in order to develop a high level of knowledge and skills in making highly contextualised, consistent and trustworthy assessment decisions to better support student learning of language.