This chapter provides an introduction to formative assessment, especially applied within an online or e-learning environment. The characteristics of four strategies of online formative assessment currently most widely used—online adaptive assessment, online self-assessment, online collaborative assessment, and portfolio—are described. References are made throughout recent research about the effectiveness of online formative assessment for optimizing students’ learning. A case study in which a computer-assisted assessment tool was used to design and apply self-assessment exercises is presented. The chapter emphasizes the idea that all type of assessment needs to be conceptualized as “assessment for learning.” Practical advices are detailed for the planning, development, implementation, and review of quality formative online assessment.