Formative assessment is considered a promising approach to improving teaching and learning, especially in the Anglo-American literature. However, empirical evidence supporting this assumption is surprisingly weak. In this chapter, we introduce the concept of formative assessment by identifying the core components of formative assessment (assessment and feedback) and describing the way we assume formative assessment (via students’ perception) affects learning processes and outcomes. Furthermore, we present the project “Conditions and Consequences of Classroom Assessment” (Co2CA), consisting of four studies in which we successively investigated the design and impact of formative assessment in mathematics instruction: (1) In a survey study, we described current practice of classroom assessment in mathematics, as perceived by teachers and students, and developed mathematical tasks as a basis for the assessment component in the following studies. (2) In an experimental study, we investigated the impact of written process-oriented feedback on learning in an internally valid setting. (3) In an intervention study, we implemented the instruments and results of the first two studies in mathematics instruction to analyze the impact of formative assessment in an ecologically valid setting. (4) Finally, we conducted a transfer study to make our results usable in educational practice: We developed a teacher training in formative assessment and investigated its impact on teachers’ general pedagogical and pedagogical content knowledge. This chapter focuses on a description of the designs and selected results of Studies 3 and 4.