The central goal of this chapter is to describe six principles that have proven to be especially useful to help researchers (or teachers, curriculum designers, or assessment specialists) create thought revealing activities that we refer to as model-eliciting activities. The first section describes general characteristics and purposes of model-eliciting activities. The second section gives examples of several model-eliciting activities, and it also describes typical solutions that students generate to such problems. The third section states several disclaimers about modeleliciting activities, which tend to cause confusion if they are not addressed early. The fourth section describes difficulties that modeleliciting activities were designed to address in research; and, the fifth section describes principles for designing productive model-eliciting activities.