A central aspect of teachers’ professional knowledge and competence is the ability to assess students’ achievements adequately. Giving grades and marks is one prototypical task in this context. Besides giving grades, assessments for school placements or tracking decisions belong to these tasks. Relevant students’ characteristics which influence teachers’ assessments do not only involve academic achievement but also students’ responses to different task demands as well as non-academic characteristics such as learning motivation or school anxiety. Closely associated with the investigation of teachers’ assessment competences and, more specifically, the investigation of conditions associated with high quality of assessments is the development and evaluation of teacher training programs to improve professional competences. In recent years, there has been considerable progress in the domain of professional teacher training; however, only a very limited number of studies are dedicated to the question to what extend training programs might offer valuable approaches to improve the quality of assessments and to implement high assessment competences. Another important field which is closely related to teachers’ competences concerns the question how teachers’ professional development is linked to students’ learning and learning outcomes. In recent years, the societal demand for evidence that teachers’ professional development will result in improved student learning outcomes is increasing. This volume brings together questions on assessment, training, and learning in the professional development of teachers which have not been fully discussed yet. The identification of these research gaps was the reason for dedicating a series of lectures given at the University of Luxembourg 2012 to the topic of professionalization of teachers in these domains. Therefore, this book contains contributions from outstanding international scholars in different academic disciplines to present ideas about open research questions concerning the domains of assessment, training, and learning in the professional development of teachers.