The lecture will clarify the key elements of assessment and present a model for distinguishing between formative and summative purposes of assessment. It will argue for increased use of formative assessment in the classroom and it will give some examples of formative assessment methods and some recommendations for their implementation, based on ASSIST-ME experiences. The importance of working with learning progressions and deciding of criteria (indicators) will be demonstrated as well as possible consequences for student motivation. Finally the presentation will discuss possibilities for combining formative and summative aspects of assessment.