Formative Assessment (FA) in L2 classrooms has been reconceptualized in theory as it is now recognized that it is locally situated, dynamic and co-constructed in classroom interaction and is as much an informal process as a formal one. This study adopts the term "informal formative assessment" to refer to any of those FA practices that are embedded into everyday learning activities and that emerge in and through classroom interaction contingently, continuously and flexibly. It is different from formal FA, which is carried out at pre-specified times through specially designed assessment instruments. Despite the reconceptualization of FA in theory, how FA emerges informally in practice in naturally-occurring classroom interaction has not been investigated adequately. Moreover, there is a gap between classroom interaction research, which does not discuss the relevance of its findings to FA practices, and classroom-based assessment research, which neglects the role of classroom interaction in assessment practices with a greater focus on formal FA practices. In order to address this gap, this conversation-analytic study illustrates how FA informally emerges as an interactional practice in an L2 classroom through the phenomenon called "Reference to a Past Learning Event" (RPLE) and claims that such assessment practices constitute an important component of Classroom Interactional Competence. RPLE occurs when the teacher contingently extends the main instructional activity to refer to language items and topics presented in a past learning event. The teacher does this in order to check students' knowledge and/or to deal with trouble sources in their learning states in and through classroom interaction. Data are presented from a corpus of video-recordings of an EFL class (55 classroom hours) in a preparatory school at a Turkish state university. The analysis shows that RPLE emerges as a practice of informal FA in teacher turns and reveals the complexity of informal FA which is not simply about providing feedback but is dynamic and co-constructed in classroom interaction. [For the complete volume, "Classroom-Based Conversation Analytic Research: Theoretical and Applied Perspectives on Pedagogy. Educational Linguistics. Volume 46," see ED660574.]