The assessment of some kinds of learning requires the observation of student performances. Examples include playing a musical instrument, dancing, using scientific apparatus, operating equipment, performing gymnastics routines, diving, problem-solving in a group, applying ball-handling skills, and participating in small group discussions. 'Performance assessment' is the assessment of students as they engage in an activity. It is the on-the-spot evaluation of performance, behaviour or interaction. This issue on the assessment of performances in the Assessment Resource Kit (ARK) magazine series contains five articles: 1. What is performance assessment? 2. Informal classroom observations; 3. Classroom performance assessment 'events'; 4. When comparability matters; 5. Designing performance assessments.