This chapter overviews educational assessment; that is, the use of assessment to improve educational processes and outcomes. Students of education need to know how experts understand assessment. Society expects, and most curriculum frameworks require, that the learning of children is monitored. Assessment must be done well so as to fulfil that requirement. This places assessment at the heart of education: curriculum, learning, and teaching intersect in assessment. Assessment requires strong, theoretically well-grounded knowledge of learning materials and sequences, instructional actions, and what it means for students to 'know' something. Much is made currently of the three prepositions of assessment—assessment for, of, and as learning. Assessments conducted during the learning process and at the end of the process are usually called 'formative' or 'assessment for learning' and 'summative' or 'assessment of learning' respectively. 'Assessment as learning' refers to assessment-like interactions between teacher and learner during instruction.