Many tasks that students are asked to do as part of their learning are cognitively demanding and require considerable concentration. As such, the ability to concentrate is critical for school readiness as early as preschool. However, concentration is not a well-defined concept in the psychology literature. In this talk, I will briefly discuss the definition of concentration as a composite state that is comprised of many cognitive functions, namely sustained attention, selective attention, appropriate cognitive load, and self-regulation in a state of activity. I will describe the development of a scale to observe preschool children’s concentration and the preliminary results from a study using this scale in Montessori classrooms. I will discuss a variety of ways that this scale can be used to capture different aspects of children’s concentration as it occurs naturally in classrooms.