This special issue is a timely initiative in developing more knowledge and deeper understanding of assessment of, as, and for teaching. The alert reader might have noticed that the title of this author's contribution is not "Formative Assessment of Teachers", but "Formative Assessment of Teacher Learning". In her opinion it is the act of teacher learning which is object to assessment in the papers, and not the teacher as a person. Assessment research has to a large extent focused on understanding assessment of and for student learning, providing the faulty impressions that assessment of learning mainly relates to pupils or students, and not to teachers. Teacher learning has in itself been widely researched, but it has, to a large extent, ignored the assessment of teacher learning. So, the current special issue is a welcome contribution to all educators with an interest in assessment. The approach taken in this contribution to the special issue is to discuss the papers in a holistic perspective instead of going into each individual paper.