While critical literacy has recently become a pervasive discourse in policy and professional materials, there remains a most surprising silence about the theory and practice of assessment of students' performances and products in critical literacy classrooms. As the title of this paper suggests, our purpose is to move towards a theory of critical literacy and assessment, with the focus of the latter being on formative assessment designed to enhance classroom learning. Specifically, our intention is not to privilege either critical literacy or formative assessment theory, but rather to critique each from the vantage point of the other. In this way, we challenge various silences to speak about: the teaching-learning contract; mastery and apprenticeship; a dialogic relationship between teacher and student; and values and evaluation. In examining current theorising of the bases of formative assessment of students and critical literacy, we identify and explore tensions in that practice of assessment, and suggest a way beyond apparent impasses. Central to the discussion is a redefining of the nature of accountability in the theory and practice of critical literacy and assessment.